1 00:00:00,090 --> 00:00:01,670 The following content is provided 2 00:00:01,670 --> 00:00:03,820 under a Creative Commons license. 3 00:00:03,820 --> 00:00:06,550 Your support will help MIT OpenCourseWare continue 4 00:00:06,550 --> 00:00:10,160 to offer high quality educational resources for free. 5 00:00:10,160 --> 00:00:12,700 To make a donation or to view additional materials 6 00:00:12,700 --> 00:00:16,620 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:16,620 --> 00:00:17,275 at ocw.mit.edu. 8 00:00:26,130 --> 00:00:28,740 PROFESSOR: And I thought that what we'd do today 9 00:00:28,740 --> 00:00:32,900 is first go over this syllabus for audition, which 10 00:00:32,900 --> 00:00:35,040 is the second part of the course. 11 00:00:35,040 --> 00:00:37,670 Just so you get an idea of what's in store for you. 12 00:00:38,800 --> 00:00:42,610 And then today's lecture will have a big part 13 00:00:42,610 --> 00:00:47,780 on sounds, which have physical properties that 14 00:00:47,780 --> 00:00:50,630 are very different than the light stimulus you guys have 15 00:00:50,630 --> 00:00:53,610 been talking about so far in the course. 16 00:00:53,610 --> 00:00:56,900 And we're going to illustrate the different types of sounds, 17 00:00:56,900 --> 00:00:59,195 very simple sounds like pure tones. 18 00:01:00,490 --> 00:01:03,450 And very complex sounds like human speech, 19 00:01:03,450 --> 00:01:06,770 which have many, many components in them. 20 00:01:07,980 --> 00:01:10,970 And then we'll get into the auditory system, 21 00:01:10,970 --> 00:01:13,295 first starting with the auditory periphery. 22 00:01:15,300 --> 00:01:18,090 And we'll talk about the three basic divisions 23 00:01:18,090 --> 00:01:22,080 of the auditory periphery, which are the outer, middle, 24 00:01:22,080 --> 00:01:23,850 and inner ear. 25 00:01:23,850 --> 00:01:25,440 And today we're really only going 26 00:01:25,440 --> 00:01:28,470 to have a chance to focus on the functions 27 00:01:28,470 --> 00:01:30,219 of the outer and the middle ears. 28 00:01:30,219 --> 00:01:32,635 And so we'll talk about the functions of those structures. 29 00:01:35,520 --> 00:01:43,050 So as far as the syllabus goes, each of the lectures 30 00:01:43,050 --> 00:01:43,840 has a title. 31 00:01:43,840 --> 00:01:47,620 So today's title, October 28th, is sound, external, middle, 32 00:01:47,620 --> 00:01:48,345 and inner ears. 33 00:01:49,530 --> 00:01:55,270 And each of the lectures has a reading or more 34 00:01:55,270 --> 00:01:56,720 accompanying it. 35 00:01:56,720 --> 00:02:00,680 And so most of the readings are from the textbook. 36 00:02:00,680 --> 00:02:02,960 So there's a textbook, Schnupp, Nelken, 37 00:02:02,960 --> 00:02:08,820 and King, which is a very good, up-to-date textbook written 38 00:02:08,820 --> 00:02:10,710 by two psychophysicists. 39 00:02:12,530 --> 00:02:14,880 And one physiologist, Israel Nelken. 40 00:02:16,410 --> 00:02:20,570 And it's written at just the right level for this class. 41 00:02:20,570 --> 00:02:23,060 That is an advanced undergraduate textbook. 42 00:02:23,060 --> 00:02:26,970 So it's pretty easy to read, or should be very easy to read. 43 00:02:26,970 --> 00:02:28,284 And it's written very well. 44 00:02:28,284 --> 00:02:29,450 These guys are good writers. 45 00:02:30,590 --> 00:02:34,740 They have many examples of auditory demonstrations 46 00:02:34,740 --> 00:02:38,180 that you can listen to just by clicking 47 00:02:38,180 --> 00:02:39,670 in the margin of the text. 48 00:02:39,670 --> 00:02:41,030 The demonstration we'll come up. 49 00:02:41,030 --> 00:02:44,020 And as you can see, after today's lectures, 50 00:02:44,020 --> 00:02:45,440 I like to give demonstrations. 51 00:02:45,440 --> 00:02:47,250 Because I like to listen to what we're 52 00:02:47,250 --> 00:02:48,970 talking about in terms of how does 53 00:02:48,970 --> 00:02:52,355 it really sound to you as a listener. 54 00:02:53,630 --> 00:02:57,370 So I'd encourage you to get that textbook. 55 00:02:57,370 --> 00:03:00,960 Now you could buy a hard copy, or if I'm not mistaken, 56 00:03:00,960 --> 00:03:03,410 Michelle you can get a free copy online. 57 00:03:03,410 --> 00:03:04,598 Is that right? 58 00:03:04,598 --> 00:03:06,056 AUDIENCE: There's an online version 59 00:03:06,056 --> 00:03:08,757 as well that you can read on [INAUDIBLE]. 60 00:03:08,757 --> 00:03:09,340 PROFESSOR: OK. 61 00:03:09,340 --> 00:03:09,839 Great. 62 00:03:09,839 --> 00:03:14,545 So if you have any trouble figuring that out, let me know. 63 00:03:14,545 --> 00:03:16,420 But I think you should easily be able to find 64 00:03:16,420 --> 00:03:17,211 the online version. 65 00:03:18,390 --> 00:03:20,320 And it should have the demonstrations 66 00:03:20,320 --> 00:03:22,845 that you can listen to with earbuds or headphones. 67 00:03:26,300 --> 00:03:30,900 There is for today these passages from the textbook. 68 00:03:30,900 --> 00:03:34,760 And then for today and for many of the lectures, 69 00:03:34,760 --> 00:03:39,490 there is another reading, which is a research paper. 70 00:03:39,490 --> 00:03:42,440 This one is by Hofman, Van Riswick, and Opstal. 71 00:03:43,530 --> 00:03:46,410 And it's titled, Relearning Sound Localization 72 00:03:46,410 --> 00:03:48,300 with New Ears. 73 00:03:48,300 --> 00:03:52,380 And this we'll talk about in class right 74 00:03:52,380 --> 00:03:53,980 at the end of today's lecture when 75 00:03:53,980 --> 00:03:56,250 we talk about the function of the outer ear, 76 00:03:56,250 --> 00:03:57,410 the pinna, so-called. 77 00:03:58,720 --> 00:04:01,050 And they did a very interesting experiment 78 00:04:01,050 --> 00:04:04,700 that addresses what is the function of your external ear. 79 00:04:04,700 --> 00:04:09,310 So people always ask me, what am I responsible 80 00:04:09,310 --> 00:04:10,470 for in these readings. 81 00:04:10,470 --> 00:04:13,240 Well this is a very specific paper. 82 00:04:13,240 --> 00:04:15,750 It has a lot of interesting research methods. 83 00:04:15,750 --> 00:04:18,696 The subjects were human volunteers. 84 00:04:20,170 --> 00:04:22,700 And there are a lot of details in there 85 00:04:22,700 --> 00:04:24,610 that are not that important. 86 00:04:24,610 --> 00:04:27,380 What I'm really focused on having you learn 87 00:04:27,380 --> 00:04:29,900 is the take-home message. 88 00:04:29,900 --> 00:04:33,320 And the take-home message from this paper 89 00:04:33,320 --> 00:04:35,575 is, what is the function of the outer ear. 90 00:04:36,760 --> 00:04:40,280 And what is this twist in the title, how can 91 00:04:40,280 --> 00:04:44,900 you relearn sound localization with different outer ear. 92 00:04:44,900 --> 00:04:46,560 We'll talk about it in class. 93 00:04:46,560 --> 00:04:50,270 But I want you to get the take-home message 94 00:04:50,270 --> 00:04:52,950 from these research studies. 95 00:04:52,950 --> 00:04:56,700 Because there sort of what we do as professionals 96 00:04:56,700 --> 00:04:57,990 in the auditory system. 97 00:04:57,990 --> 00:05:00,310 Our day-to-day living is doing research. 98 00:05:00,310 --> 00:05:03,930 In some cases on human subjects, in some cases 99 00:05:03,930 --> 00:05:04,933 on individual molecules. 100 00:05:06,050 --> 00:05:08,530 But how can we learn about hearing 101 00:05:08,530 --> 00:05:10,420 from doing these research studies? 102 00:05:10,420 --> 00:05:16,270 And I have picked good papers, good research studies. 103 00:05:16,270 --> 00:05:17,920 Because they really tell us something. 104 00:05:17,920 --> 00:05:19,480 There's plenty of stuff out there 105 00:05:19,480 --> 00:05:22,350 that gives sort of equivocal results. 106 00:05:22,350 --> 00:05:23,940 But this is a really good paper. 107 00:05:23,940 --> 00:05:27,530 And you have a take-home point from it 108 00:05:27,530 --> 00:05:32,040 about how you use your outer ears to localize sounds. 109 00:05:32,040 --> 00:05:34,850 So that's an example of a research paper 110 00:05:34,850 --> 00:05:37,220 that goes along with this lecture. 111 00:05:37,220 --> 00:05:43,040 So just coursing through the syllabus-- on Wednesday, 112 00:05:43,040 --> 00:05:44,775 we'll have a lecture on hair cells. 113 00:05:47,900 --> 00:05:51,820 Next week we'll talk about the auditory nerve, which 114 00:05:51,820 --> 00:05:54,960 is the nerve that sends hearing information from your ear 115 00:05:54,960 --> 00:05:55,745 into your brain. 116 00:05:56,830 --> 00:06:00,320 And we'll talk about frequency resolution-- 117 00:06:00,320 --> 00:06:02,540 how we can tell one frequency from another. 118 00:06:03,925 --> 00:06:05,300 At the end of next week, we'll be 119 00:06:05,300 --> 00:06:09,600 talking about the brain, the cochlear nucleus, and all 120 00:06:09,600 --> 00:06:12,595 the interesting unit and cell types in the cochlear nucleus. 121 00:06:14,310 --> 00:06:16,210 The following week, we're going to be talking 122 00:06:16,210 --> 00:06:18,970 about hearing loss, how there can 123 00:06:18,970 --> 00:06:20,310 be problems with your hearing. 124 00:06:20,310 --> 00:06:22,450 Many of them are treated at the hospital where 125 00:06:22,450 --> 00:06:25,850 I do my research, which is Massachusetts Eye and Ear 126 00:06:25,850 --> 00:06:27,380 Infirmary across the river. 127 00:06:28,510 --> 00:06:33,820 And there, when the surgeons in my department 128 00:06:33,820 --> 00:06:35,550 encounter a deaf person, they give them 129 00:06:35,550 --> 00:06:38,470 the option to get a cochlear implant. 130 00:06:38,470 --> 00:06:40,690 So a cochlear implant is a device 131 00:06:40,690 --> 00:06:42,680 that can be put in your inner ear. 132 00:06:42,680 --> 00:06:45,420 And it can restore your sense of hearing. 133 00:06:45,420 --> 00:06:48,260 And we'll have a demonstration by that cochlear implant 134 00:06:48,260 --> 00:06:51,120 user who comes to class on that date 135 00:06:51,120 --> 00:06:55,180 and gives a demonstration of her cochlear implant, which 136 00:06:55,180 --> 00:06:57,505 has restored hearing to her, although not perfectly. 137 00:06:59,530 --> 00:07:01,420 So then later on in the semester, 138 00:07:01,420 --> 00:07:06,800 we'll talk about various other topics 139 00:07:06,800 --> 00:07:08,460 on up through the auditory cortex. 140 00:07:10,020 --> 00:07:15,130 And finally, we're going to have a tour of the Hearing Research 141 00:07:15,130 --> 00:07:17,930 Laboratory at the Massachusetts Eye and Ear Infirmary, 142 00:07:17,930 --> 00:07:21,270 where we'll meet over there and we'll 143 00:07:21,270 --> 00:07:23,590 encounter various research projects 144 00:07:23,590 --> 00:07:26,047 that are currently going on. 145 00:07:26,047 --> 00:07:27,130 And we'll talk about them. 146 00:07:28,690 --> 00:07:30,780 There is a written assignment. 147 00:07:30,780 --> 00:07:33,390 I guess you guys had an assignment for vision 148 00:07:33,390 --> 00:07:35,460 in the class-- a written paper? 149 00:07:35,460 --> 00:07:37,970 So we have an analog here on the auditory system. 150 00:07:39,030 --> 00:07:42,540 And this is the assignment you can read it later 151 00:07:42,540 --> 00:07:43,370 at your leisure. 152 00:07:43,370 --> 00:07:46,180 It won't make much sense right now, 153 00:07:46,180 --> 00:07:48,710 because we haven't talked about neural circuits 154 00:07:48,710 --> 00:07:50,880 for localization of sounds yet. 155 00:07:50,880 --> 00:07:52,120 You can look on the syllabus. 156 00:07:52,120 --> 00:07:55,840 It's about halfway through the second part of the class. 157 00:07:56,860 --> 00:07:58,950 And there's a lot of details here. 158 00:07:58,950 --> 00:08:04,130 And it asks you what's updated since an original model was 159 00:08:04,130 --> 00:08:08,650 postulated by a researcher called [? Jeffrus ?]. 160 00:08:08,650 --> 00:08:11,600 So that's a paper-- I don't think 161 00:08:11,600 --> 00:08:13,940 I said how long it should be. 162 00:08:13,940 --> 00:08:16,250 How long was the paper for vision? 163 00:08:16,250 --> 00:08:16,960 Was there a link? 164 00:08:18,535 --> 00:08:19,410 AUDIENCE: [INAUDIBLE] 165 00:08:19,410 --> 00:08:20,770 PROFESSOR: Four to six pages? 166 00:08:20,770 --> 00:08:23,165 OK, four pages sounds good. 167 00:08:23,165 --> 00:08:25,820 If you really want to write six, you probably could. 168 00:08:29,070 --> 00:08:31,480 But we'll talk about this when we 169 00:08:31,480 --> 00:08:34,020 talk about sound localization in the class. 170 00:08:34,020 --> 00:08:37,480 And I think the due date here is written. 171 00:08:37,480 --> 00:08:39,210 It's the date of the lab tour. 172 00:08:42,100 --> 00:08:44,340 And then, we have a final exam in the class. 173 00:08:44,340 --> 00:08:47,010 And I think as Doctor Schiller talked about 174 00:08:47,010 --> 00:08:49,440 at the very first day, the final exam 175 00:08:49,440 --> 00:08:52,890 will be waited toward the auditory system, 176 00:08:52,890 --> 00:08:56,181 which we haven't had a test on by the time this exam rolls 177 00:08:56,181 --> 00:08:56,680 around. 178 00:08:56,680 --> 00:09:00,340 So I think it's going to be 2/3 audition 179 00:09:00,340 --> 00:09:02,960 on the final exam, and 1/3 vision. 180 00:09:02,960 --> 00:09:05,070 And there are several review sessions 181 00:09:05,070 --> 00:09:08,960 for both senses planned at the end of the semester. 182 00:09:11,210 --> 00:09:13,822 So any questions about the organization 183 00:09:13,822 --> 00:09:15,340 of what we're going to do? 184 00:09:18,130 --> 00:09:21,100 OK, so I'll start today's lecture. 185 00:09:30,780 --> 00:09:34,910 And I think the PowerPoint files-- for today's lecture 186 00:09:34,910 --> 00:09:38,280 and all the rest of the lectures for the rest of the semester 187 00:09:38,280 --> 00:09:40,260 are available in the course website. 188 00:09:40,260 --> 00:09:45,380 So you can look at them now or as the lecture comes up. 189 00:09:46,390 --> 00:09:48,820 So first, we're going to talk about 190 00:09:48,820 --> 00:09:51,820 the physical characteristics of sound-- 191 00:09:51,820 --> 00:09:54,450 just very, very different than the characteristics 192 00:09:54,450 --> 00:09:55,755 of the light stimulus. 193 00:09:56,880 --> 00:10:02,550 And maybe light stimuli are so obvious that Peter Schiller 194 00:10:02,550 --> 00:10:05,450 probably didn't spend much time in his lecture about it. 195 00:10:05,450 --> 00:10:08,450 But I'm going to spend 10 or 15 minutes here 196 00:10:08,450 --> 00:10:10,290 on the physical characteristics of sound. 197 00:10:10,290 --> 00:10:13,720 Because it's very different than the light stimulus. 198 00:10:13,720 --> 00:10:19,880 So sound is a mechanical, radiated energy, 199 00:10:19,880 --> 00:10:24,660 transmitted by longitudinal vibrations of a m 200 00:10:24,660 --> 00:10:27,485 so you have to have a medium to transmit sound. 201 00:10:28,520 --> 00:10:30,490 You can have light go through outer space 202 00:10:30,490 --> 00:10:31,415 in a complete vacuum. 203 00:10:32,630 --> 00:10:36,379 But in outer space, you can't have sound 204 00:10:36,379 --> 00:10:37,795 because you have to have a medium. 205 00:10:39,180 --> 00:10:42,510 The medium can be various types of things. 206 00:10:42,510 --> 00:10:44,690 We're going to talk mostly about sound in air. 207 00:10:45,840 --> 00:10:49,830 But you could have sound in water-- whales make songs, 208 00:10:49,830 --> 00:10:54,120 and they sing to each other, and one whale listens to another. 209 00:10:54,120 --> 00:10:56,640 And in between the two is a medium of water. 210 00:10:56,640 --> 00:11:01,670 You can have sound in a solid-- if you live in an apartment 211 00:11:01,670 --> 00:11:06,330 room, and you hear your neighbors' music, you 212 00:11:06,330 --> 00:11:07,500 especially here the base. 213 00:11:07,500 --> 00:11:11,090 Because the low frequency sound transmits pretty well 214 00:11:11,090 --> 00:11:14,380 through solids, like the solid of the wall 215 00:11:14,380 --> 00:11:15,635 in between the two apartments. 216 00:11:17,600 --> 00:11:21,240 Sound can go in many, many different types of media. 217 00:11:21,240 --> 00:11:26,220 In air, like we're going to use mostly for this course, 218 00:11:26,220 --> 00:11:28,430 you can think of sound is being produced 219 00:11:28,430 --> 00:11:32,630 by a sound source like the piston of your loudspeaker. 220 00:11:33,960 --> 00:11:37,240 And the piston goes back and forth. . 221 00:11:37,240 --> 00:11:40,840 It's driven back and forth by an electric voltage 222 00:11:40,840 --> 00:11:44,720 and when it goes this way, it presses on the air molecules 223 00:11:44,720 --> 00:11:46,440 in front of it. 224 00:11:46,440 --> 00:11:49,640 And it presses them so they're closer together 225 00:11:49,640 --> 00:11:51,795 and makes them a little bit higher in pressure. 226 00:11:53,150 --> 00:11:58,330 And that's what's meant by this compression or condensation. 227 00:11:58,330 --> 00:12:01,140 And these dots close together means a little bit 228 00:12:01,140 --> 00:12:02,335 of an area of high pressure. 229 00:12:03,800 --> 00:12:06,730 Then as the piston moves the other direction, 230 00:12:06,730 --> 00:12:08,590 it rarefies the air. 231 00:12:08,590 --> 00:12:11,020 It drags some of the air with it. 232 00:12:11,020 --> 00:12:13,320 And so that little space right in front of the piston 233 00:12:13,320 --> 00:12:15,140 has a lower pressure. 234 00:12:15,140 --> 00:12:19,210 Because there are fewer molecules per volume 235 00:12:19,210 --> 00:12:19,765 than before. 236 00:12:21,060 --> 00:12:28,420 So this energy, then, is transmitted through the medium 237 00:12:28,420 --> 00:12:32,700 to whatever-- a microphone, which detects sound, 238 00:12:32,700 --> 00:12:35,290 or a listener, which can listen to the sound. 239 00:12:37,290 --> 00:12:41,870 If you have a microphone or some kind of detector that 240 00:12:41,870 --> 00:12:45,390 can plot that pressure at any one point-- 241 00:12:45,390 --> 00:12:47,312 let's say the microphone is right at the edge 242 00:12:47,312 --> 00:12:47,895 of this paper. 243 00:12:48,910 --> 00:12:53,980 And you graph the pressure as a function of time on this graph. 244 00:12:53,980 --> 00:12:56,385 So here's pressure and here's time. 245 00:12:57,590 --> 00:13:02,040 As those radiated energy wave fronts pass you, 246 00:13:02,040 --> 00:13:03,860 the pressure will go up. 247 00:13:03,860 --> 00:13:05,070 And then it will go down. 248 00:13:05,070 --> 00:13:07,150 And then it will go up and go down. 249 00:13:07,150 --> 00:13:10,045 And it will repeat over and over as long as that piston moves. 250 00:13:12,030 --> 00:13:15,500 So this horizontal line is simply 251 00:13:15,500 --> 00:13:20,140 the barometric or static pressure of the air. 252 00:13:20,140 --> 00:13:23,210 And sure, the barometric pressure changes a little bit. 253 00:13:23,210 --> 00:13:27,060 If there's a hurricane coming, it gets way low. 254 00:13:27,060 --> 00:13:30,100 If there's a high pressure like we have right now-- 255 00:13:30,100 --> 00:13:32,260 sunny climate, the barometric pressure goes up. 256 00:13:33,300 --> 00:13:35,186 But those are very slow fluctuations. 257 00:13:36,350 --> 00:13:41,340 And the sound wave form is a very, very fast waveform 258 00:13:41,340 --> 00:13:43,960 that goes many times per second. 259 00:13:44,990 --> 00:13:48,580 In fact, what we call as sound frequency 260 00:13:48,580 --> 00:13:53,945 is the number of oscillations of that pressure wave per second. 261 00:13:55,530 --> 00:13:58,650 And they are very fast. 262 00:13:58,650 --> 00:14:03,480 As you can see down here on this so-called audiogram 263 00:14:03,480 --> 00:14:06,820 or frequency curve for human hearing, 264 00:14:06,820 --> 00:14:10,290 the frequencies, which are on the x-axis here, 265 00:14:10,290 --> 00:14:19,490 go from 10 Hertz-- Hertz means cycles per second-- 266 00:14:19,490 --> 00:14:26,250 so one Hertz is one cycle per second. 267 00:14:28,540 --> 00:14:30,270 And that is a frequency that's so 268 00:14:30,270 --> 00:14:31,610 low it didn't get on this graph. 269 00:14:32,930 --> 00:14:34,750 Because humans aren't sensitive to 270 00:14:34,750 --> 00:14:37,970 frequencies that slow or that low. 271 00:14:37,970 --> 00:14:40,550 Usually the lower limit for human hearing 272 00:14:40,550 --> 00:14:44,640 is considered to be about 10 cycles per second, or 10 Hertz. 273 00:14:44,640 --> 00:14:51,290 And it extends all the way up to 20,000 cycles per second. 274 00:14:51,290 --> 00:14:52,910 And in the middle of the human range, 275 00:14:52,910 --> 00:14:54,440 we'll be talking about hearing a lot 276 00:14:54,440 --> 00:14:57,640 at a middle frequency of about 1,000 Hertz. 277 00:14:57,640 --> 00:15:01,890 So that's a nice, round, middle frequency for you 278 00:15:01,890 --> 00:15:03,900 to remember for human hearing. 279 00:15:05,810 --> 00:15:08,620 So we're talking about pressure oscillations 280 00:15:08,620 --> 00:15:11,900 in terms of thousands of times per second, 281 00:15:11,900 --> 00:15:13,436 or hundreds of times per second. 282 00:15:13,436 --> 00:15:14,310 So they're very fast. 283 00:15:16,360 --> 00:15:19,020 There will be examples during our course 284 00:15:19,020 --> 00:15:22,210 where the auditory system-- the auditory neurons 285 00:15:22,210 --> 00:15:24,660 keep track of those cycles, even though they're 286 00:15:24,660 --> 00:15:28,160 going back and forth thousands of times per second. 287 00:15:29,710 --> 00:15:31,710 So we'll come back to that in future lectures. 288 00:15:34,530 --> 00:15:38,000 Now this is the audiogram for human hearing 289 00:15:38,000 --> 00:15:39,280 in the solid curve here. 290 00:15:39,280 --> 00:15:42,290 This is supposed to say human, if you could read it. 291 00:15:42,290 --> 00:15:48,350 And on the y-axis is how strong the stimulus 292 00:15:48,350 --> 00:15:55,250 is, how loud it is, or in terms of physical characteristics, 293 00:15:55,250 --> 00:15:58,390 what the sound pressure is. 294 00:15:58,390 --> 00:16:01,950 And this scale goes from minus 20 to 140. 295 00:16:01,950 --> 00:16:06,150 And the units are dB SPL and that 296 00:16:06,150 --> 00:16:10,605 stands for decibels sound pressure level. 297 00:16:12,220 --> 00:16:14,600 And whenever you hear level in a formula, 298 00:16:14,600 --> 00:16:18,910 you should perk up your ears and say oh, that 299 00:16:18,910 --> 00:16:22,170 means there's a log-- a logarithm-- in the formula. 300 00:16:22,170 --> 00:16:25,790 And sure enough, the formula for a sound pressure level 301 00:16:25,790 --> 00:16:31,124 is 20 times the log of whatever sound pressure 302 00:16:31,124 --> 00:16:32,540 you're talking about, whatever you 303 00:16:32,540 --> 00:16:37,350 were listening to or measured by your microphone divided 304 00:16:37,350 --> 00:16:38,980 by some reference pressure. 305 00:16:40,520 --> 00:16:41,860 That's the formula. 306 00:16:41,860 --> 00:16:43,520 And the reference pressure is given 307 00:16:43,520 --> 00:16:47,670 as 20 micronewtons per square meter. 308 00:16:47,670 --> 00:16:50,040 OK, so let's figure that out. 309 00:16:50,040 --> 00:16:53,660 What is Newton a unit of? 310 00:16:53,660 --> 00:16:54,160 Anybody? 311 00:16:54,160 --> 00:16:54,826 AUDIENCE: Force. 312 00:16:54,826 --> 00:16:58,950 PROFESSOR: Right, force-- and meter squared is area. 313 00:16:58,950 --> 00:17:03,645 So we're talking about force per area, and that's pressure. 314 00:17:05,609 --> 00:17:09,060 So Newton obviously was like Hertz, one of the people 315 00:17:09,060 --> 00:17:10,380 who was interested in physics. 316 00:17:12,079 --> 00:17:17,010 And a Newton is a unit of force per square meter is pressure. 317 00:17:17,010 --> 00:17:21,380 Now in more modern terms, the unit micronewton 318 00:17:21,380 --> 00:17:23,369 per square meter has been renamed 319 00:17:23,369 --> 00:17:33,340 to be a pascal, abbreviated Pa. 320 00:17:35,630 --> 00:17:36,530 So it's the same. 321 00:17:36,530 --> 00:17:41,630 One Pascal is one Newton per square meter. 322 00:17:41,630 --> 00:17:44,100 In this case, we're talking about micro-- Newtons 323 00:17:44,100 --> 00:17:45,875 are micro Pascals. 324 00:17:51,540 --> 00:17:57,520 So why is that number chosen as the reference 325 00:17:57,520 --> 00:18:00,070 for this very important sound pressure level scale? 326 00:18:00,070 --> 00:18:04,975 Well, it's actually chosen with the hearing system in mind. 327 00:18:06,910 --> 00:18:11,930 What they did in the 1930s, when this was being developed, 328 00:18:11,930 --> 00:18:14,500 is they rounded up a bunch of people at a county fair, 329 00:18:14,500 --> 00:18:16,600 gave them headphones, and said we're 330 00:18:16,600 --> 00:18:20,330 going to try a nice mid-frequency. 331 00:18:20,330 --> 00:18:23,200 Let's try 1,000 Hertz. 332 00:18:23,200 --> 00:18:25,110 They gave them a tone at 1,000 Hertz. 333 00:18:25,110 --> 00:18:27,290 The listeners listened to it. 334 00:18:27,290 --> 00:18:29,430 Then they said I can hear that fine. 335 00:18:29,430 --> 00:18:31,930 Then they turned the level down a little bit. 336 00:18:31,930 --> 00:18:33,930 And the person said yeah, I can still hear that. 337 00:18:33,930 --> 00:18:36,980 Then they turned it down so much that the person didn't say, 338 00:18:36,980 --> 00:18:38,530 I hear something. 339 00:18:38,530 --> 00:18:40,060 There were silent. 340 00:18:40,060 --> 00:18:41,840 They turned it up a little-- says yeah, 341 00:18:41,840 --> 00:18:43,730 I hear-- They turned it down. 342 00:18:43,730 --> 00:18:47,320 They titrated the levels until it was right at threshold, 343 00:18:47,320 --> 00:18:48,555 just barely detectable. 344 00:18:50,280 --> 00:18:53,420 And they took an average of 30-some people. 345 00:18:53,420 --> 00:18:56,240 And they said that is going to be 346 00:18:56,240 --> 00:18:59,910 the basis of our sound pressure level scale. 347 00:18:59,910 --> 00:19:03,940 So it's actually a term that was derived biologically 348 00:19:03,940 --> 00:19:05,810 by testing people's hearing. 349 00:19:08,250 --> 00:19:11,870 So that's kind of a nice story. 350 00:19:11,870 --> 00:19:13,030 I wonder if it's true. 351 00:19:13,030 --> 00:19:14,780 Well, let's look at it. 352 00:19:14,780 --> 00:19:22,930 Where does the human hearing curve, that 1,000 Hertz, fall? 353 00:19:22,930 --> 00:19:28,850 Where should it fall if 20 micronewtons per square meter 354 00:19:28,850 --> 00:19:30,510 is the pressure you're talking about? 355 00:19:30,510 --> 00:19:32,440 It's the same as the reference pressure. 356 00:19:32,440 --> 00:19:34,050 What's 20 over 20? 357 00:19:34,050 --> 00:19:35,360 It's 1. 358 00:19:35,360 --> 00:19:36,705 What's the log of 1? 359 00:19:41,040 --> 00:19:41,730 Zero. 360 00:19:41,730 --> 00:19:42,600 Correct. 361 00:19:42,600 --> 00:19:47,432 20 times the log of 1 is 0-- sound pressure level 0. 362 00:19:47,432 --> 00:19:48,890 Well, look at our curve right here, 363 00:19:48,890 --> 00:19:52,540 that 1,000 Hertz-- it's pretty close to 0. 364 00:19:53,570 --> 00:19:55,920 Why might it not be exactly zero? 365 00:19:55,920 --> 00:19:58,014 Well the people that were used for this curve 366 00:19:58,014 --> 00:20:00,555 were a little bit different than the ones in the county fair. 367 00:20:02,300 --> 00:20:05,615 We'll study later on that some people have a hearing loss. 368 00:20:06,950 --> 00:20:10,354 Hearing can be affected by the room that you used. 369 00:20:10,354 --> 00:20:12,270 Maybe there was a lot of yelling and screaming 370 00:20:12,270 --> 00:20:13,225 at the county fair. 371 00:20:15,350 --> 00:20:17,545 We have better rooms to test hearing now. 372 00:20:18,730 --> 00:20:22,140 It turns out that the human hearing curve is actually 373 00:20:22,140 --> 00:20:26,720 a little more sensitive at 2,000, 3,000, and maybe 4,000. 374 00:20:26,720 --> 00:20:31,510 So when the pressures go below the reference pressure, 375 00:20:31,510 --> 00:20:34,030 the number becomes less than 1. 376 00:20:34,030 --> 00:20:36,620 And the logarithm becomes negative. 377 00:20:36,620 --> 00:20:40,290 It's perfectly fine to have a negative SPL. 378 00:20:40,290 --> 00:20:45,500 We have some points on the graph for that-- minus 2, minus 3 dB. 379 00:20:46,720 --> 00:20:51,630 This other dashed audiogram, or hearing sensitivity curve, 380 00:20:51,630 --> 00:20:53,890 is for a different species-- the cat. 381 00:20:53,890 --> 00:20:56,770 And the cat here's down to about minus 10 382 00:20:56,770 --> 00:21:00,490 dB SPL-- at least this group of cats did. 383 00:21:00,490 --> 00:21:04,210 The cats also hear higher in frequency than humans. 384 00:21:05,350 --> 00:21:07,650 Dogs and cats can hear about an octave 385 00:21:07,650 --> 00:21:10,730 higher-- that is a doubling of frequency 386 00:21:10,730 --> 00:21:13,200 higher than humans do, and maybe some of you 387 00:21:13,200 --> 00:21:15,990 have had dog whistles that you blow. 388 00:21:15,990 --> 00:21:17,210 And you don't hear anything. 389 00:21:17,210 --> 00:21:21,760 But the dog comes because it's a very high frequency 390 00:21:21,760 --> 00:21:24,280 beyond the upper limit of human hearing, 391 00:21:24,280 --> 00:21:28,275 but well within the hearing range of those species. 392 00:21:30,900 --> 00:21:33,760 So different species have different hearing ranges. 393 00:21:37,854 --> 00:21:38,775 AUDIENCE: Professor? 394 00:21:38,775 --> 00:21:39,400 PROFESSOR: Yes. 395 00:21:39,400 --> 00:21:42,352 AUDIENCE: Sorry-- just to clarify, 396 00:21:42,352 --> 00:21:46,288 is a micropascal then [INAUDIBLE]? 397 00:21:46,288 --> 00:21:47,280 PROFESSOR: No. 398 00:21:47,280 --> 00:21:54,070 These are units of pressure-- micronewtons per square meter-- 399 00:21:54,070 --> 00:21:56,395 and this is a unit of pressure. 400 00:21:58,310 --> 00:22:02,455 SPL is just in these units called decibels. 401 00:22:03,740 --> 00:22:05,769 And it it's not a pressure-- 402 00:22:05,769 --> 00:22:07,060 AUDIENCE: It's the log of that. 403 00:22:07,060 --> 00:22:07,600 PROFESSOR: That's right. 404 00:22:07,600 --> 00:22:08,675 It's the log of that. 405 00:22:10,767 --> 00:22:11,600 Any other questions? 406 00:22:16,000 --> 00:22:20,700 So these are sort of the lower limits of hearing. 407 00:22:20,700 --> 00:22:26,020 When you go into conversational levels, or the level of a lawn 408 00:22:26,020 --> 00:22:30,870 mower, or the level of a concert, 409 00:22:30,870 --> 00:22:33,390 the levels get higher-- still certainly 410 00:22:33,390 --> 00:22:35,850 within your audibility range. 411 00:22:35,850 --> 00:22:38,700 As you go to a higher and higher level, 412 00:22:38,700 --> 00:22:41,190 you risk damage to your hearing. 413 00:22:41,190 --> 00:22:45,690 And at that risk level, which it says high risk thresholds here. 414 00:22:45,690 --> 00:22:50,790 And right around 120 dB, sounds become painfully 415 00:22:50,790 --> 00:22:54,400 loud and damaging to your hearing. 416 00:22:54,400 --> 00:22:58,600 And that's what this shaded area refers 417 00:22:58,600 --> 00:23:00,960 to-- gunshots, jet aircraft engine. 418 00:23:00,960 --> 00:23:03,760 And we'll talk about that during our lecture of hearing loss. 419 00:23:09,580 --> 00:23:10,933 So I have some demonstrations. 420 00:23:14,020 --> 00:23:17,880 Because a lot of people have trouble with the decibel scale. 421 00:23:17,880 --> 00:23:19,350 So what is a decibel? 422 00:23:19,350 --> 00:23:23,840 And what does it sound like when you change the sound from 50 423 00:23:23,840 --> 00:23:26,820 dB to 60 dB? 424 00:23:26,820 --> 00:23:28,710 Well this demonstration has three parts. 425 00:23:29,880 --> 00:23:32,670 And let me read the text first. 426 00:23:34,060 --> 00:23:39,120 Broadband noise-- sometimes it's called white noise. 427 00:23:39,120 --> 00:23:41,840 Broadband noise and white noise are synonyms. 428 00:23:41,840 --> 00:23:45,337 And what is white light as a visual stimulus? 429 00:23:45,337 --> 00:23:46,420 AUDIENCE: All wavelengths. 430 00:23:46,420 --> 00:23:48,350 PROFESSOR: All wavelengths, right? 431 00:23:48,350 --> 00:23:51,862 And so broadband noise means it has all frequencies. 432 00:23:51,862 --> 00:23:55,140 It has 10 Hertz, 20 Hertz, 30 Hertz, 1,000 433 00:23:55,140 --> 00:23:57,565 Hertz, 2,000-- it has all frequencies. 434 00:23:58,620 --> 00:24:01,270 And it sounds like the "shh" sound. 435 00:24:03,220 --> 00:24:08,420 So you hear this "shh." it'll start out pretty loud. 436 00:24:08,420 --> 00:24:15,220 It'll be reduced in ten steps of six decibels for each step. 437 00:24:15,220 --> 00:24:18,211 And I think you'll be able to very clearly hear 438 00:24:18,211 --> 00:24:20,460 the difference between the first and the second steps. 439 00:24:21,680 --> 00:24:24,500 And demonstrations are repeated once. 440 00:24:24,500 --> 00:24:27,900 The second demonstration is same noise 441 00:24:27,900 --> 00:24:31,680 is reduced in 15 steps-- now of three decibels. 442 00:24:31,680 --> 00:24:35,290 So this is a little bit of a smaller scale, 443 00:24:35,290 --> 00:24:37,240 though you'll still be clearly audible. 444 00:24:38,580 --> 00:24:40,450 Third, broadband noise is reduced 445 00:24:40,450 --> 00:24:42,880 in 20 steps of now one dB. 446 00:24:42,880 --> 00:24:46,255 So let's listen to see if we can hear 1 dB steps. 447 00:24:49,568 --> 00:24:52,526 RECORDING: The decibel scale-- broadband 448 00:24:52,526 --> 00:24:57,949 noise is reduced in 10 sets of 6 decibels. 449 00:24:57,949 --> 00:25:00,907 [INAUDIBLE] repeated once. 450 00:25:00,907 --> 00:25:12,290 [TONE] [TONE] 451 00:25:12,290 --> 00:25:14,787 OK, was that clear-- the difference 452 00:25:14,787 --> 00:25:15,870 between one and the other? 453 00:25:15,870 --> 00:25:17,920 So that's what 6 dB sounds like? 454 00:25:17,920 --> 00:25:21,780 Now, you guys who are up here close to the speakers, 455 00:25:21,780 --> 00:25:26,240 you might be starting at 85 dB SPL on the first ones-- pretty 456 00:25:26,240 --> 00:25:27,540 loud. 457 00:25:27,540 --> 00:25:28,976 6 dB lower is 79. 458 00:25:30,390 --> 00:25:31,780 And then, so on and so forth. 459 00:25:31,780 --> 00:25:33,960 You guys at the back are further from the speaker. 460 00:25:33,960 --> 00:25:36,190 You're not starting at the same level. 461 00:25:36,190 --> 00:25:39,520 You might be starting at 60 dB. 462 00:25:39,520 --> 00:25:43,700 You're still going down 6 dB to 54 dB in the next step. 463 00:25:43,700 --> 00:25:45,200 Everything is linear in here. 464 00:25:45,200 --> 00:25:47,980 It doesn't matter where you start from, 465 00:25:47,980 --> 00:25:50,150 as long as you're going down 6 dB. 466 00:25:50,150 --> 00:25:53,830 So where you start doesn't really matter in these demos. 467 00:25:53,830 --> 00:25:55,790 RECORDING: Broadbad noise is reduced 468 00:25:55,790 --> 00:25:59,710 in 15 steps of 3 decibels. 469 00:25:59,710 --> 00:26:16,880 [TONE] [TONE] 470 00:26:16,880 --> 00:26:18,817 PROFESSOR: OK, still clear the increment 471 00:26:18,817 --> 00:26:19,900 between one and the other? 472 00:26:19,900 --> 00:26:22,480 OK, now here's the one dB steps. 473 00:26:22,480 --> 00:26:24,021 RECORDING: Broadband noise is reduced 474 00:26:24,021 --> 00:26:26,967 in 20 steps of one decibel. 475 00:26:28,440 --> 00:26:52,150 [TONE] [TONE] 476 00:26:52,150 --> 00:26:54,270 PROFESSOR: OK so how about for that? 477 00:26:54,270 --> 00:26:56,740 Would you be able to stake your life on the fact 478 00:26:56,740 --> 00:26:58,700 that you could tell one from another? 479 00:26:58,700 --> 00:27:00,110 No, I see a lot of heads shaking. 480 00:27:01,210 --> 00:27:05,390 Well if you sit there and do this over and over again, 481 00:27:05,390 --> 00:27:09,870 and really train yourself, apparently 1 dB 482 00:27:09,870 --> 00:27:11,950 is the just noticeable difference 483 00:27:11,950 --> 00:27:14,665 that most observers can here. 484 00:27:16,230 --> 00:27:27,465 So 1 dB is the just noticeable difference in SPL. 485 00:27:48,790 --> 00:27:50,860 So how do we do that? 486 00:27:50,860 --> 00:27:53,490 Well you have an auditory nerve. 487 00:27:53,490 --> 00:27:57,740 And at 60 dB, your auditory nerve fibers 488 00:27:57,740 --> 00:27:59,730 are sending this many spikes to the brain. 489 00:27:59,730 --> 00:28:05,340 At 61 dB, they're sending maybe a few more spikes-- something 490 00:28:05,340 --> 00:28:05,840 like that. 491 00:28:05,840 --> 00:28:08,040 It's not absolutely clear how you do that. 492 00:28:08,040 --> 00:28:12,060 There is more information coming in from the ear to the brain 493 00:28:12,060 --> 00:28:13,580 as a function on sound level. 494 00:28:13,580 --> 00:28:14,746 We'll talk a lot about that. 495 00:28:15,490 --> 00:28:20,915 Now, we also talked about sound frequency. 496 00:28:22,750 --> 00:28:25,640 JND for sound level is about 1 dB. 497 00:28:25,640 --> 00:28:27,110 What is it for sound frequency? 498 00:28:27,110 --> 00:28:29,960 We're going to have pretty much a whole lecture on that. 499 00:28:31,010 --> 00:28:35,620 But your ear is extremely good at telling one frequency 500 00:28:35,620 --> 00:28:37,000 from another. 501 00:28:37,000 --> 00:28:44,390 So if you start at 1,000 Hertz and change it to 1,002 502 00:28:44,390 --> 00:28:47,560 Hertz-- very, very small change-- 503 00:28:47,560 --> 00:28:49,522 you can tell the difference. 504 00:28:49,522 --> 00:28:52,435 Your ear is a fantastic frequency analyzer. 505 00:28:53,442 --> 00:28:54,900 We're going to have a whole lecture 506 00:28:54,900 --> 00:28:57,440 on exactly how your ear does that. 507 00:28:57,440 --> 00:29:01,490 But the JND for sound frequency is also a good demonstration. 508 00:29:01,490 --> 00:29:05,560 We'll play that when we talk about sound frequency coding. 509 00:29:07,551 --> 00:29:08,050 OK. 510 00:29:08,050 --> 00:29:09,715 Any questions about that so far? 511 00:29:14,980 --> 00:29:15,860 OK. 512 00:29:15,860 --> 00:29:20,075 Let's switch back to the physical characteristics 513 00:29:20,075 --> 00:29:20,575 of sound. 514 00:29:21,740 --> 00:29:24,140 And these are some very common auditory stimuli. 515 00:29:25,560 --> 00:29:28,140 We've heard a noise just now. 516 00:29:28,140 --> 00:29:33,450 And if you graph the sound pressure as a function of time, 517 00:29:33,450 --> 00:29:35,470 this is what the waveform looks like. 518 00:29:36,520 --> 00:29:37,730 How could you do that? 519 00:29:37,730 --> 00:29:39,670 If you take a microphone, stick it out 520 00:29:39,670 --> 00:29:43,970 in front of a noise source, and run that into an oscilloscope, 521 00:29:43,970 --> 00:29:48,650 the microphone converts the sound pressure into a voltage, 522 00:29:48,650 --> 00:29:51,900 the oscilloscope displays the voltage signal 523 00:29:51,900 --> 00:29:52,960 as a function of time. 524 00:29:52,960 --> 00:29:54,102 You can look at that. 525 00:29:55,580 --> 00:29:58,150 Auditory scientists like to look at things 526 00:29:58,150 --> 00:30:00,750 as a function of time, of course. 527 00:30:00,750 --> 00:30:02,770 They also like to look at things as a function 528 00:30:02,770 --> 00:30:04,275 of sound frequency. 529 00:30:07,240 --> 00:30:12,400 This is a graph for this same stimulus, a noise stimulus, 530 00:30:12,400 --> 00:30:14,320 now as a function of frequency. 531 00:30:14,320 --> 00:30:17,890 And we said before, the noise is broadband. 532 00:30:17,890 --> 00:30:18,790 It's white noise. 533 00:30:18,790 --> 00:30:20,250 It has all frequencies. 534 00:30:20,250 --> 00:30:22,640 And here is the graph to show you that. 535 00:30:22,640 --> 00:30:25,330 This might be the energy, and this 536 00:30:25,330 --> 00:30:27,310 is as a function of frequency. 537 00:30:27,310 --> 00:30:28,590 So it has all frequencies. 538 00:30:28,590 --> 00:30:31,400 It's trailing off a little at the very highest. 539 00:30:31,400 --> 00:30:34,450 That may be because the microphone couldn't wiggle 540 00:30:34,450 --> 00:30:37,490 back and forth at very, very high frequencies. 541 00:30:37,490 --> 00:30:40,690 But it's essentially a flat frequency curve. 542 00:30:40,690 --> 00:30:44,213 And sometimes this display is called the spectrum. 543 00:30:50,250 --> 00:30:54,724 So spectrum or spectra are graphs as a function 544 00:30:54,724 --> 00:30:55,265 of frequency. 545 00:30:56,750 --> 00:30:59,000 Sometimes people talk about this as a frequency 546 00:30:59,000 --> 00:31:01,940 domain and the time domain. 547 00:31:01,940 --> 00:31:04,960 If you've taken any electrical engineering courses here 548 00:31:04,960 --> 00:31:09,660 at MIT, people will talk about the time and frequency domains. 549 00:31:09,660 --> 00:31:12,920 And how can you go from one representation to another? 550 00:31:12,920 --> 00:31:14,720 Well, you can take your microphone signal 551 00:31:14,720 --> 00:31:16,710 instead of going to the oscilloscope, 552 00:31:16,710 --> 00:31:18,860 going to the spectrum analyzer, which 553 00:31:18,860 --> 00:31:21,660 is a machine that can give you this nice plot. 554 00:31:21,660 --> 00:31:23,390 But how about mathematically? 555 00:31:23,390 --> 00:31:24,500 How can you do that? 556 00:31:25,860 --> 00:31:27,460 The Fourier Transform, right. 557 00:31:41,750 --> 00:31:45,470 Of course, Fourier was a mathematician 558 00:31:45,470 --> 00:31:50,450 who studied various things, heat transfer and other things. 559 00:31:50,450 --> 00:31:52,030 He developed this transformation. 560 00:31:52,030 --> 00:31:54,240 If you have the mathematical description 561 00:31:54,240 --> 00:31:57,290 of a time-varying signal, you can 562 00:31:57,290 --> 00:32:00,790 plug it through his equation, the Fourier transform, 563 00:32:00,790 --> 00:32:04,520 and come out with the frequency representation or the frequency 564 00:32:04,520 --> 00:32:05,020 domain. 565 00:32:06,070 --> 00:32:08,740 Or, vice versa, if you have the frequency domain, 566 00:32:08,740 --> 00:32:10,729 you can inverse Fourier transform and go back 567 00:32:10,729 --> 00:32:11,520 to the time domain. 568 00:32:13,470 --> 00:32:16,140 We're not going to talk too much about transforms here. 569 00:32:16,140 --> 00:32:21,140 But it is interesting, because, as it turns out, 570 00:32:21,140 --> 00:32:25,910 your inner ear is a wonderful frequency analyzer. 571 00:32:25,910 --> 00:32:30,350 It can tell the difference between 1,000 and 1,002 Hertz. 572 00:32:30,350 --> 00:32:33,380 This is a very nice way in the ear 573 00:32:33,380 --> 00:32:35,170 of detecting the different frequencies. 574 00:32:35,170 --> 00:32:39,780 And so these time and frequency domain representations 575 00:32:39,780 --> 00:32:41,720 are very convenient for us to look at. 576 00:32:41,720 --> 00:32:43,010 So just keep that in mind. 577 00:32:44,780 --> 00:32:50,570 Here's a very common auditory stimulus, the pure tone 578 00:32:50,570 --> 00:32:51,330 or the sinusoid. 579 00:32:53,470 --> 00:32:56,910 This is a sinusoidal waveform in the time domain. 580 00:32:56,910 --> 00:33:00,866 In the frequency domain, it only has one frequency-- 581 00:33:00,866 --> 00:33:02,240 the frequency at which that thing 582 00:33:02,240 --> 00:33:05,050 is going back and forth in terms of Hertz. 583 00:33:06,510 --> 00:33:08,230 This is in a Hertz axis. 584 00:33:09,240 --> 00:33:12,050 So sometimes it's called a pure tone. 585 00:33:12,050 --> 00:33:13,060 Why is it so pure? 586 00:33:14,080 --> 00:33:16,230 Does it have high morals or what? 587 00:33:16,230 --> 00:33:19,255 No, it just has one sound frequency. 588 00:33:21,910 --> 00:33:24,900 These other stimuli, we're going to listen 589 00:33:24,900 --> 00:33:26,110 to this in just a minute. 590 00:33:26,110 --> 00:33:27,945 This is a so-called square wave. 591 00:33:31,160 --> 00:33:37,910 Imagine trying to add up a whole bunch of pure tones 592 00:33:37,910 --> 00:33:40,030 to result in a square wave. 593 00:33:40,030 --> 00:33:41,660 It seems impossible, right? 594 00:33:42,780 --> 00:33:45,770 Well, it's possible if you use an infinite number 595 00:33:45,770 --> 00:33:46,570 of frequencies. 596 00:33:46,570 --> 00:33:48,690 And this frequency representation 597 00:33:48,690 --> 00:33:52,550 for a square wave goes on basically forever. 598 00:33:52,550 --> 00:33:55,090 To get those corners of the square wave 599 00:33:55,090 --> 00:33:58,380 sharp like a true square wave, you 600 00:33:58,380 --> 00:34:01,560 need lots of individual frequencies, 601 00:34:01,560 --> 00:34:03,770 lots of pure tones, if you will. 602 00:34:06,360 --> 00:34:10,468 Tone bursts are some common auditory stimuli. 603 00:34:10,468 --> 00:34:12,259 We'll talk about those later in the course. 604 00:34:13,560 --> 00:34:16,710 Click is a very common auditory stimulus. 605 00:34:16,710 --> 00:34:18,860 It's a sound like this. 606 00:34:18,860 --> 00:34:23,250 Or last night, it was the sound of a fastball 607 00:34:23,250 --> 00:34:24,724 hitting a wooden baseball bat. 608 00:34:26,530 --> 00:34:30,210 It's a very sharp, impulsive sound, very nice sound 609 00:34:30,210 --> 00:34:32,400 if you're behind the team who's batting. 610 00:34:34,489 --> 00:34:37,300 So a click, that baseball hitting the bat, 611 00:34:37,300 --> 00:34:39,290 doesn't happen for very long. 612 00:34:39,290 --> 00:34:43,539 A click can be infinitesimally short. 613 00:34:45,210 --> 00:34:47,480 The time that the baseball is in contact with the bat 614 00:34:47,480 --> 00:34:48,409 is pretty short. 615 00:34:49,929 --> 00:34:52,780 And if it's very short in the time domain, 616 00:34:52,780 --> 00:34:54,394 then you have all frequencies. 617 00:34:55,870 --> 00:35:00,290 So it's another example of a broadband or broad spectrum 618 00:35:00,290 --> 00:35:01,590 sound. 619 00:35:01,590 --> 00:35:04,310 If the click is infinitesimally short, 620 00:35:04,310 --> 00:35:06,580 the spectrum is completely flat. 621 00:35:09,510 --> 00:35:11,240 Those are some common auditory stimuli. 622 00:35:14,720 --> 00:35:19,870 Let's go through some more complicated, 623 00:35:19,870 --> 00:35:22,170 and maybe more interesting, sounds. 624 00:35:22,170 --> 00:35:25,420 Well, all of us like to listen to music, right? 625 00:35:25,420 --> 00:35:28,900 So here are some examples of musical sounds. 626 00:35:28,900 --> 00:35:31,006 This is a piano keyboard. 627 00:35:32,790 --> 00:35:38,000 And here is the spectrum or frequency representation 628 00:35:38,000 --> 00:35:43,290 of what you get when you strike one key on the piano keyboard. 629 00:35:43,290 --> 00:35:44,470 So that's one note. 630 00:35:46,360 --> 00:35:48,890 Well, sure, it sounds like one thing, 631 00:35:48,890 --> 00:35:52,910 but you have a whole bunch of different frequencies 632 00:35:52,910 --> 00:35:54,160 that go along with it. 633 00:35:54,160 --> 00:35:55,640 And why is that true? 634 00:35:55,640 --> 00:35:56,390 Does anybody know? 635 00:35:56,390 --> 00:35:58,598 Why do you get a whole bunch of different frequencies 636 00:35:58,598 --> 00:36:01,550 when you strike a key on the piano keyboard? 637 00:36:04,389 --> 00:36:04,889 Yeah? 638 00:36:04,889 --> 00:36:07,857 AUDIENCE: Isn't it vibrating all along the length 639 00:36:07,857 --> 00:36:12,330 so there's different wavelengths? 640 00:36:12,330 --> 00:36:13,555 PROFESSOR: What's vibrating-- 641 00:36:13,555 --> 00:36:14,735 AUDIENCE: It's not-- 642 00:36:14,735 --> 00:36:15,200 PROFESSOR: In the piano? 643 00:36:15,200 --> 00:36:17,650 AUDIENCE: It's not-- it's like an infinitely small portion 644 00:36:17,650 --> 00:36:19,210 of the string. 645 00:36:20,470 --> 00:36:22,812 It's the longer string. 646 00:36:22,812 --> 00:36:28,660 It's parts that are shorter still vibrating. 647 00:36:28,660 --> 00:36:31,240 PROFESSOR: Yeah, you're getting there. 648 00:36:31,240 --> 00:36:35,470 In the piano, the string is fixed at one end, 649 00:36:35,470 --> 00:36:37,415 and it's a long string. 650 00:36:37,415 --> 00:36:39,030 It [? fits ?] [? in ?] the other. 651 00:36:39,030 --> 00:36:43,900 And your key that you press down makes a hammer go up, 652 00:36:43,900 --> 00:36:45,340 and there's a bunch of linkages. 653 00:36:45,340 --> 00:36:48,450 And eventually, the hammer hits that string somewhere. 654 00:36:49,980 --> 00:36:51,970 And the string, it's fixed here. 655 00:36:51,970 --> 00:36:53,620 It's not going to move. 656 00:36:53,620 --> 00:36:54,370 It's fixed here. 657 00:36:54,370 --> 00:36:55,400 It's not going to move. 658 00:36:55,400 --> 00:36:58,670 But in between those points, it can move. 659 00:36:58,670 --> 00:37:04,070 So it can vibrate like this, or it can go up and down. 660 00:37:06,410 --> 00:37:08,240 It can also vibrate like this. 661 00:37:14,430 --> 00:37:16,860 You can have what's called a node in the middle. 662 00:37:18,300 --> 00:37:24,310 In fact, if you put your finger right here and fix that middle, 663 00:37:24,310 --> 00:37:26,470 it wouldn't allow the string to vibrate 664 00:37:26,470 --> 00:37:28,570 in this uniform fashion. 665 00:37:28,570 --> 00:37:31,460 But it would allow this half to vibrate and that half 666 00:37:31,460 --> 00:37:32,210 to vibrate. 667 00:37:32,210 --> 00:37:35,795 This node is sort of a constraint for this string. 668 00:37:35,795 --> 00:37:37,675 It can also vibrate like this. 669 00:37:38,739 --> 00:37:40,030 I wish I had a different color. 670 00:37:41,820 --> 00:37:42,497 Over here? 671 00:37:42,497 --> 00:37:42,997 Great. 672 00:37:48,530 --> 00:37:49,030 OK. 673 00:37:49,030 --> 00:37:56,930 You can also have the string vibrate like this. 674 00:37:59,265 --> 00:37:59,765 OK. 675 00:38:02,110 --> 00:38:05,010 And it can vibrate in many, many different patterns. 676 00:38:05,010 --> 00:38:06,700 I've just drawn a few. 677 00:38:06,700 --> 00:38:10,580 What's interesting is that this length is twice 678 00:38:10,580 --> 00:38:15,990 as long as this length, which is twice as long as this length. 679 00:38:15,990 --> 00:38:20,150 And what would you expect the time of those vibrations to be? 680 00:38:20,150 --> 00:38:23,920 Well, the big long thing is going to vibrate pretty slowly. 681 00:38:26,592 --> 00:38:28,550 That's what's called the fundamental frequency. 682 00:38:30,480 --> 00:38:32,970 The thing that's vibrating in two parts, it's shorter 683 00:38:32,970 --> 00:38:34,175 and it can vibrate faster. 684 00:38:35,690 --> 00:38:39,000 In fact, it vibrates twice as fast. 685 00:38:39,000 --> 00:38:44,000 So the first harmonic is twice the frequency 686 00:38:44,000 --> 00:38:49,040 of the fundamental, and so on and so forth. 687 00:38:49,040 --> 00:38:51,820 You can get from the physical characteristics 688 00:38:51,820 --> 00:38:55,710 of the vibration of that string a whole bunch 689 00:38:55,710 --> 00:38:58,320 of different vibration patterns. 690 00:38:58,320 --> 00:39:03,120 And they're usually a harmonic series-- twice, three times, 691 00:39:03,120 --> 00:39:07,880 four times, five times, six times-- the fundamental, 692 00:39:07,880 --> 00:39:10,630 just because of the physical characteristics of vibration 693 00:39:10,630 --> 00:39:14,320 of the string, and the wind column 694 00:39:14,320 --> 00:39:16,260 in the case of an Alto saxophone. 695 00:39:18,930 --> 00:39:23,730 When you hear that one note hit by the hammer, 696 00:39:23,730 --> 00:39:27,110 all of these vibrations are happening at once. 697 00:39:27,110 --> 00:39:32,910 And so that one sound sounds like one thing. 698 00:39:32,910 --> 00:39:38,527 Musicians will say it sounds like a note-- A above C. 699 00:39:38,527 --> 00:39:40,860 But you have a whole bunch of different harmonics in it. 700 00:39:45,150 --> 00:39:46,000 What is pitch? 701 00:39:47,410 --> 00:39:51,490 Pitch is very interesting to people 702 00:39:51,490 --> 00:39:54,320 who study the auditory system, to musicians. 703 00:39:54,320 --> 00:39:59,520 Pitch is that attribute of the sensation, auditory sensation, 704 00:39:59,520 --> 00:40:02,580 in terms of which sounds can be ordered on a musical scale. 705 00:40:04,080 --> 00:40:07,300 Let's say I didn't let you see the keyboard, 706 00:40:07,300 --> 00:40:10,910 but I recorded the sounds, and I press some sounds down there, 707 00:40:10,910 --> 00:40:14,870 some in the middle, some way up here, some way at the high end, 708 00:40:14,870 --> 00:40:17,145 and I gave you 20 different recordings, 709 00:40:17,145 --> 00:40:21,040 and I said, well, make a ranking of them. 710 00:40:21,040 --> 00:40:22,300 Put these down low. 711 00:40:22,300 --> 00:40:23,680 Those are number one and two. 712 00:40:24,720 --> 00:40:26,470 Put these in the middle-- those are number 713 00:40:26,470 --> 00:40:28,810 10-- up to the high end. 714 00:40:28,810 --> 00:40:30,390 The highest one is 20. 715 00:40:30,390 --> 00:40:31,435 You could do that. 716 00:40:33,020 --> 00:40:34,740 The ones that were down low would 717 00:40:34,740 --> 00:40:36,350 be called those with low pitch. 718 00:40:39,250 --> 00:40:41,120 The pitch of a pure tone, of course, 719 00:40:41,120 --> 00:40:43,080 depends on the frequency. 720 00:40:43,080 --> 00:40:45,240 That's as if you were just giving one. 721 00:40:46,410 --> 00:40:48,870 If you move that around, up high end frequency, 722 00:40:48,870 --> 00:40:52,020 it sounds like a really shrilly, high-pitched sound. 723 00:40:52,020 --> 00:40:54,430 If you move it down low, it sounds like a real low sound. 724 00:40:56,700 --> 00:41:00,280 The pitch of a complicated sound-- that is, 725 00:41:00,280 --> 00:41:03,150 with many overtones and harmonics-- 726 00:41:03,150 --> 00:41:06,170 depends strongly on the fundamental frequency. 727 00:41:06,170 --> 00:41:09,150 But sometimes, the fundamental-- for example, in this guitar 728 00:41:09,150 --> 00:41:10,135 sound-- is pretty weak. 729 00:41:11,510 --> 00:41:14,220 And in some cases, you can take it out altogether. 730 00:41:14,220 --> 00:41:16,640 The pitch doesn't change that much, surprisingly. 731 00:41:18,540 --> 00:41:21,835 So somehow, the ear knows by this pattern of spectrum 732 00:41:21,835 --> 00:41:23,960 that there should be a fundamental [INAUDIBLE] that 733 00:41:23,960 --> 00:41:24,834 can stick it back in. 734 00:41:27,040 --> 00:41:29,140 So that's what pitch is. 735 00:41:29,140 --> 00:41:32,210 Another sensation that musicians often talk about 736 00:41:32,210 --> 00:41:34,050 is the timbre of a sound. 737 00:41:34,050 --> 00:41:38,295 And the timbre is the quality or the identification of a sound. 738 00:41:39,620 --> 00:41:42,060 It relates to the highest harmonics here 739 00:41:42,060 --> 00:41:44,340 and the pattern of this harmonics. 740 00:41:44,340 --> 00:41:47,610 For the piano, it's starting big and sloping down. 741 00:41:47,610 --> 00:41:51,050 For a guitar, it's starting small, sloping up, 742 00:41:51,050 --> 00:41:53,130 and then sloping down. 743 00:41:53,130 --> 00:41:58,210 The timbre is what allows you to identify that sound that you 744 00:41:58,210 --> 00:41:59,270 heard as a piano. 745 00:41:59,270 --> 00:42:02,062 We can all hear a piano and say, that's a piano. 746 00:42:02,062 --> 00:42:04,145 We can all hear a guitar and say, that's a guitar, 747 00:42:04,145 --> 00:42:06,960 or that's an electric guitar, because its pattern 748 00:42:06,960 --> 00:42:11,220 of harmonics, its fundamental harmonics, differs. 749 00:42:11,220 --> 00:42:15,210 That's how we identify sounds is by their timbre 750 00:42:15,210 --> 00:42:16,960 or their spectrum, if you will. 751 00:42:21,090 --> 00:42:23,410 Those are pretty complicated sounds. 752 00:42:26,220 --> 00:42:27,150 What do I have next? 753 00:42:27,150 --> 00:42:28,108 I have a demonstration. 754 00:42:30,380 --> 00:42:33,590 This one is called Canceled Harmonics. 755 00:42:33,590 --> 00:42:36,340 And it's a very nice demonstration 756 00:42:36,340 --> 00:42:39,370 to illustrate the idea that I said, 757 00:42:39,370 --> 00:42:42,350 when you have all these harmonics go on together, 758 00:42:42,350 --> 00:42:46,449 it sounds like one thing, one note, one sound. 759 00:42:46,449 --> 00:42:47,990 But if you take some of the harmonics 760 00:42:47,990 --> 00:42:53,220 out and put them back in, you're aware of that 761 00:42:53,220 --> 00:42:55,680 taking out and putting back in. 762 00:42:55,680 --> 00:42:59,530 So what they're going to do is a complex tone 763 00:42:59,530 --> 00:43:03,510 is presented, followed by several cancellations 764 00:43:03,510 --> 00:43:07,210 and restorations of a particular harmonic. 765 00:43:07,210 --> 00:43:09,477 And let me show you what complex tones 766 00:43:09,477 --> 00:43:10,560 they're going to give you. 767 00:43:12,990 --> 00:43:14,925 It's simply this square wave. 768 00:43:16,986 --> 00:43:18,860 This is what you're going to be listening to. 769 00:43:18,860 --> 00:43:20,401 It sounds like [MAKES BUZZING NOISE]. 770 00:43:20,401 --> 00:43:22,060 It's not very musical at all. 771 00:43:23,770 --> 00:43:25,830 And it has a fundamental and a whole bunch 772 00:43:25,830 --> 00:43:27,490 of harmonics, an infinite number. 773 00:43:29,680 --> 00:43:32,850 When that complex goes on at once, you're going to say, 774 00:43:32,850 --> 00:43:35,730 that sounds like a nasty sound. 775 00:43:35,730 --> 00:43:37,165 It sounds like a buzz almost. 776 00:43:38,450 --> 00:43:44,150 Then they're going to take this one harmonic and pull it out, 777 00:43:44,150 --> 00:43:45,960 and then they're going to put it back in. 778 00:43:47,380 --> 00:43:48,999 As they do that, you're going to say, 779 00:43:48,999 --> 00:43:50,290 well, that sounded differently. 780 00:43:51,460 --> 00:43:53,510 When it was out and when it was back in, 781 00:43:53,510 --> 00:43:55,390 I could hear that thing going in and out. 782 00:43:55,390 --> 00:43:58,600 And then they're going to do that for the second, third, 783 00:43:58,600 --> 00:44:01,340 and fourth on up to, I think about 10 or so. 784 00:44:02,540 --> 00:44:04,470 Even though this whole constellation 785 00:44:04,470 --> 00:44:09,920 sounds like one sound, when they pulse these things in and out, 786 00:44:09,920 --> 00:44:11,740 you can tell. 787 00:44:11,740 --> 00:44:13,410 Let's listen to the demonstration, 788 00:44:13,410 --> 00:44:15,800 and let's see how many times they're going to do it. 789 00:44:15,800 --> 00:44:19,255 This is done for harmonics one through ten. 790 00:44:26,855 --> 00:44:28,322 Canceled Harmonics. 791 00:44:28,322 --> 00:44:31,010 A complex tone is presented, followed 792 00:44:31,010 --> 00:44:34,160 by several cancellations and restorations 793 00:44:34,160 --> 00:44:36,136 of a particular harmonic. 794 00:44:36,136 --> 00:44:39,594 This is done for harmonics one through 10. 795 00:46:05,502 --> 00:46:06,030 OK. 796 00:46:06,030 --> 00:46:09,330 Could everybody hear when this complex went on all at 797 00:46:09,330 --> 00:46:11,780 once it sounded like one sound? 798 00:46:11,780 --> 00:46:15,600 Then when individual components were taken out and pulsed back 799 00:46:15,600 --> 00:46:17,490 in, you could identify them. 800 00:46:17,490 --> 00:46:20,640 Your ear is very good at distinguishing 801 00:46:20,640 --> 00:46:22,825 the various frequencies in a complex spectrum. 802 00:46:24,190 --> 00:46:27,360 All that message is sent to the brain as individual channels, 803 00:46:27,360 --> 00:46:29,920 and the brain somehow perceives that when everything 804 00:46:29,920 --> 00:46:33,910 is going on at the same time, that's one sound. 805 00:46:33,910 --> 00:46:35,610 It's really not of interest to the brain 806 00:46:35,610 --> 00:46:38,270 that the string is vibrating a whole bunch 807 00:46:38,270 --> 00:46:40,090 of different frequencies. 808 00:46:40,090 --> 00:46:41,985 It's that there's one string vibrating. 809 00:46:43,530 --> 00:46:46,550 But if you took out one of these modes-- in other words, 810 00:46:46,550 --> 00:46:50,420 if I put my finger here and the fundamental goes away, 811 00:46:50,420 --> 00:46:53,350 you ear is very good at detecting that. 812 00:46:53,350 --> 00:46:55,170 And it sends a message to the brain 813 00:46:55,170 --> 00:46:56,970 that the fundamental is no longer there. 814 00:46:56,970 --> 00:46:59,178 And the brain says, something different has happened. 815 00:47:00,670 --> 00:47:03,700 So the ear is very good at recognizing 816 00:47:03,700 --> 00:47:05,260 those different characteristics. 817 00:47:05,260 --> 00:47:07,720 The brain is good at putting them back together and saying, 818 00:47:07,720 --> 00:47:10,194 they started at one time, so it's one object. 819 00:47:13,290 --> 00:47:14,680 Questions about that so far? 820 00:47:18,790 --> 00:47:24,920 Now, the last type of complex sound that I want to cover 821 00:47:24,920 --> 00:47:26,040 is speech sounds. 822 00:47:26,040 --> 00:47:29,530 And I want to save most of this for the end of the semester 823 00:47:29,530 --> 00:47:34,700 when we talk about the parts of the auditory system that 824 00:47:34,700 --> 00:47:38,220 are active in distinguishing different speech sounds. 825 00:47:38,220 --> 00:47:41,140 But let me just-- because we're talking about sounds 826 00:47:41,140 --> 00:47:44,520 and complex sounds, talk about speech sounds. 827 00:47:44,520 --> 00:47:46,730 This is a diagram of your vocal cavity. 828 00:47:48,100 --> 00:47:50,530 Way down at the bottom here, you get air 829 00:47:50,530 --> 00:47:53,410 from your lungs that goes through your trachea. 830 00:47:53,410 --> 00:47:57,430 And in the trachea, there's these vocal cords, if you will, 831 00:47:57,430 --> 00:48:00,190 that are scientifically called the glottis. 832 00:48:02,060 --> 00:48:05,890 The opening in between is the glottis. 833 00:48:05,890 --> 00:48:09,670 So air can come out, or if you use muscles associated 834 00:48:09,670 --> 00:48:11,910 with your vocal cords, you can close that off. 835 00:48:14,650 --> 00:48:21,010 As the air comes out from here, it moves those vocal cords 836 00:48:21,010 --> 00:48:22,230 back and forth. 837 00:48:22,230 --> 00:48:24,540 And they hit each other, and they open up, 838 00:48:24,540 --> 00:48:26,060 and they hit each other and open up. 839 00:48:27,310 --> 00:48:30,210 And as they do that, they interrupt the airflow 840 00:48:30,210 --> 00:48:31,642 and they allow it to pass through. 841 00:48:31,642 --> 00:48:34,100 And they interrupt it, and they allowed it to pass through. 842 00:48:35,340 --> 00:48:38,720 And if you were to put a microphone 843 00:48:38,720 --> 00:48:43,340 way down your trachea right above those vocal cords, 844 00:48:43,340 --> 00:48:45,980 you would see this time waveform. 845 00:48:45,980 --> 00:48:50,340 The pressure would go up right as the air pressure is coming 846 00:48:50,340 --> 00:48:52,650 from the lungs when the vocal cords were open. 847 00:48:53,720 --> 00:48:57,620 When the vocal cords are shut, there's no pressure there, 848 00:48:57,620 --> 00:49:00,580 or it's just atmospheric pressure. 849 00:49:00,580 --> 00:49:05,210 So this opening and closing of the air through the glottis 850 00:49:05,210 --> 00:49:08,450 forms this very complicated waveform. 851 00:49:08,450 --> 00:49:11,344 If you look at the spectrum of it, 852 00:49:11,344 --> 00:49:13,260 it has a whole bunch of different frequencies. 853 00:49:14,530 --> 00:49:16,890 The lowest of the frequencies is the frequency 854 00:49:16,890 --> 00:49:19,050 that these things are opening and closing. 855 00:49:19,050 --> 00:49:21,180 But there's a whole bunch of harmonics. 856 00:49:21,180 --> 00:49:22,675 It's a very complicated spectrum. 857 00:49:25,200 --> 00:49:27,620 The upper part of your vocal tract 858 00:49:27,620 --> 00:49:28,820 is what's called the filter. 859 00:49:30,730 --> 00:49:35,180 And it serves to emphasize some of those harmonics 860 00:49:35,180 --> 00:49:36,715 and de-emphasize others. 861 00:49:37,800 --> 00:49:41,070 And the filter function is indicated here 862 00:49:41,070 --> 00:49:42,765 having three peaks. 863 00:49:43,910 --> 00:49:46,130 Those peaks are called formant peaks. 864 00:49:58,620 --> 00:50:03,090 They have to do with the shape and dimensions, lengths 865 00:50:03,090 --> 00:50:06,310 and widths of your upper vocal tract. 866 00:50:06,310 --> 00:50:10,980 What's kind of neat is by manipulating, let's say, 867 00:50:10,980 --> 00:50:15,320 where your palate is, and where your lips are, 868 00:50:15,320 --> 00:50:19,160 and where your tongue is, you can change that filter function 869 00:50:19,160 --> 00:50:21,350 by using the muscles that move things around 870 00:50:21,350 --> 00:50:24,000 in your upper vocal tract. 871 00:50:24,000 --> 00:50:27,600 And after you've filtered this complex spectrum, 872 00:50:27,600 --> 00:50:30,780 you come out with a function where 873 00:50:30,780 --> 00:50:34,560 some of these spectral peaks are emphasized 874 00:50:34,560 --> 00:50:36,077 and some are not emphasized. 875 00:50:36,077 --> 00:50:37,660 And here's the function that you would 876 00:50:37,660 --> 00:50:40,990 get right outside in the air outside the front 877 00:50:40,990 --> 00:50:42,400 of your mouth. 878 00:50:42,400 --> 00:50:44,045 This is the time wave form here. 879 00:50:46,800 --> 00:50:51,140 Here are some examples of manipulation 880 00:50:51,140 --> 00:50:53,480 of your upper vocal tract. 881 00:50:53,480 --> 00:50:55,720 For instance, here the lower part of the mouth 882 00:50:55,720 --> 00:51:01,310 is moved way up high, and it produces an acoustic spectrum 883 00:51:01,310 --> 00:51:02,600 where you have a big f1. 884 00:51:04,120 --> 00:51:06,630 And f2 and f3 are small, and they 885 00:51:06,630 --> 00:51:08,080 are way up high in frequency. 886 00:51:10,600 --> 00:51:15,560 Contrast this with when the bottom of your mouth 887 00:51:15,560 --> 00:51:17,370 is lowered and moved backward. 888 00:51:18,560 --> 00:51:21,160 Here, F1 is even lower. 889 00:51:21,160 --> 00:51:23,500 F2 is quite low. 890 00:51:23,500 --> 00:51:25,330 And F3 is moderately low. 891 00:51:25,330 --> 00:51:27,100 And these are, of course, the way 892 00:51:27,100 --> 00:51:29,860 you pronounce different vowels. 893 00:51:31,470 --> 00:51:33,300 We can all say these two vowels. 894 00:51:33,300 --> 00:51:36,640 This is the vowel "i" as in "hit." 895 00:51:36,640 --> 00:51:39,640 Everybody say that-- hit, hit. 896 00:51:39,640 --> 00:51:44,790 You can kind of feel that the lower part of your mouth 897 00:51:44,790 --> 00:51:46,400 is moved upward. 898 00:51:46,400 --> 00:51:50,544 Whereas if you do something like this-- "a" in call. 899 00:51:50,544 --> 00:51:52,120 Call-- everybody say that. 900 00:51:52,120 --> 00:51:53,120 Call. 901 00:51:53,120 --> 00:51:54,930 You can feel the lower part of your mouth 902 00:51:54,930 --> 00:51:59,690 dropping down as indicated here in making a big cavity, 903 00:51:59,690 --> 00:52:02,110 whereas here the cavity is very small. 904 00:52:02,110 --> 00:52:04,090 It changes the acoustic spectrum. 905 00:52:04,090 --> 00:52:05,510 Our ears pick it up. 906 00:52:06,730 --> 00:52:09,960 And our ears are very good frequency analyzers. 907 00:52:09,960 --> 00:52:13,730 And they say the spectrum here sounds like hit, because you've 908 00:52:13,730 --> 00:52:17,830 learned to associate that spectrum with that vowel. 909 00:52:17,830 --> 00:52:19,220 This is a different spectrum. 910 00:52:19,220 --> 00:52:20,970 Our ears pick it up and they say, 911 00:52:20,970 --> 00:52:25,580 that's the vowel "a" as in "call." 912 00:52:25,580 --> 00:52:29,300 That's how speech sounds are formed. 913 00:52:29,300 --> 00:52:33,010 At least this works very well for vowel sounds. 914 00:52:33,010 --> 00:52:37,280 It doesn't explain things like consonant sounds, which 915 00:52:37,280 --> 00:52:39,220 of course are many different kinds. 916 00:52:39,220 --> 00:52:42,520 There's stop consonants where your lips close down 917 00:52:42,520 --> 00:52:45,570 before you utter the consonant "p." 918 00:52:45,570 --> 00:52:49,220 So "p," everybody close their lips down, 919 00:52:49,220 --> 00:52:51,124 and then all of the sudden you open it up. 920 00:52:51,124 --> 00:52:52,540 It's a completely different thing. 921 00:52:52,540 --> 00:52:54,860 That's not modulating the spectrum. 922 00:52:54,860 --> 00:52:57,420 That's modulating the time pattern. 923 00:52:57,420 --> 00:53:00,500 These vowels are distinguished by 924 00:53:00,500 --> 00:53:02,520 their different spectral patterns, 925 00:53:02,520 --> 00:53:04,530 which is picked up by your years. 926 00:53:04,530 --> 00:53:06,850 So I just thought you'd want to know about that. 927 00:53:06,850 --> 00:53:11,420 Speech sounds are among the most complicated acoustical sounds 928 00:53:11,420 --> 00:53:14,420 because of the number of frequencies involved, 929 00:53:14,420 --> 00:53:17,320 the formation, and of course the perception 930 00:53:17,320 --> 00:53:20,850 of telling, for example, one vowel from another. 931 00:53:26,100 --> 00:53:30,240 Let's shift gears and move on. 932 00:53:30,240 --> 00:53:32,730 And instead of talking about the physical characteristics 933 00:53:32,730 --> 00:53:35,240 of sound, let's talk about how we hear sounds. 934 00:53:36,950 --> 00:53:40,280 We're only going to get as far as the auditory periphery 935 00:53:40,280 --> 00:53:42,820 today, but let's just define it. 936 00:53:42,820 --> 00:53:47,260 The auditory periphery is this whole structure indicated here, 937 00:53:47,260 --> 00:53:50,230 and it's usually separated into three parts-- 938 00:53:50,230 --> 00:53:58,190 the external ear, the middle ear, and then the inner ear. 939 00:53:58,190 --> 00:54:01,910 Those are the three very big divisions 940 00:54:01,910 --> 00:54:03,610 of the auditory periphery. 941 00:54:03,610 --> 00:54:05,625 In the external ear, you have your pinna. 942 00:54:07,170 --> 00:54:08,730 Here's your pinna. 943 00:54:08,730 --> 00:54:12,030 You have the ear canal, which goes down 944 00:54:12,030 --> 00:54:15,190 about three centimeters inside your head, 945 00:54:15,190 --> 00:54:18,020 and it ends up at this yellow structure here called 946 00:54:18,020 --> 00:54:19,370 the ear drum. 947 00:54:19,370 --> 00:54:22,190 Tympanic membrane is the scientific term 948 00:54:22,190 --> 00:54:23,010 for the ear drum. 949 00:54:24,400 --> 00:54:27,000 That's the end of the external ear. 950 00:54:27,000 --> 00:54:31,810 The middle ear is an air-filled cavity. 951 00:54:31,810 --> 00:54:34,860 So we're still talking about sound in the ear. 952 00:54:34,860 --> 00:54:38,930 In that middle ear cavity are three small bones. 953 00:54:40,220 --> 00:54:42,130 They're called ossicles. 954 00:54:42,130 --> 00:54:43,640 I think-- yeah, here we go. 955 00:54:44,730 --> 00:54:46,930 And in high school biology, you probably 956 00:54:46,930 --> 00:54:52,410 learned them as hammer, anvil, and stirrup. 957 00:54:52,410 --> 00:54:57,070 But the scientific names are malleus, incus, and stapes. 958 00:55:00,300 --> 00:55:05,030 And they convey these sound vibrations of the ear drum. 959 00:55:05,030 --> 00:55:08,230 When sound hits the ear drum, it causes it to move. 960 00:55:08,230 --> 00:55:12,460 And these bones are linked right onto the eardrum, 961 00:55:12,460 --> 00:55:13,925 and they're linked one to another. 962 00:55:15,070 --> 00:55:18,030 The ear drum then moves the bones, 963 00:55:18,030 --> 00:55:22,390 and the bones finally end up, in the case of the stapes, 964 00:55:22,390 --> 00:55:23,624 in the inner ear. 965 00:55:23,624 --> 00:55:25,165 So that's where the inner ear begins. 966 00:55:27,050 --> 00:55:30,380 I have a demonstration of ossicles, 967 00:55:30,380 --> 00:55:33,700 and I'll pass them around. 968 00:55:33,700 --> 00:55:36,880 These are ossicles from a guinea pig, 969 00:55:36,880 --> 00:55:39,090 and they're glued to the bottom of this little vial. 970 00:55:41,040 --> 00:55:43,510 And I made a crummy drawing of them. 971 00:55:44,980 --> 00:55:49,190 But if you hold this vial so that the piece of tape on it 972 00:55:49,190 --> 00:55:53,380 is downward, you get this view here. 973 00:55:53,380 --> 00:55:54,240 You have the stapes. 974 00:55:55,930 --> 00:55:58,270 And I didn't list the other ones. 975 00:55:59,470 --> 00:56:07,170 But in the guinea pig, the incus and malleus 976 00:56:07,170 --> 00:56:10,010 are fused, so they can be considered one. 977 00:56:11,620 --> 00:56:14,150 This is definitely part of the malleus. 978 00:56:17,010 --> 00:56:22,670 But I don't know where the incus ends and the malleus begins. 979 00:56:22,670 --> 00:56:27,010 If you had an ear drum, it would be this dashed line here. 980 00:56:32,010 --> 00:56:33,590 So let me just pass these around. 981 00:56:35,170 --> 00:56:39,900 And you can probably appreciate from my diagram 982 00:56:39,900 --> 00:56:44,700 how the high school biology name for the stapes got its name. 983 00:56:44,700 --> 00:56:45,849 It's the stirrup. 984 00:56:45,849 --> 00:56:46,640 What's the stirrup? 985 00:56:53,330 --> 00:56:55,390 Does anybody know what a stirrup is? 986 00:56:56,781 --> 00:56:57,280 Yeah. 987 00:56:57,280 --> 00:56:58,972 When you ride horses, what is the-- 988 00:56:58,972 --> 00:57:00,388 AUDIENCE: You put your foot in it. 989 00:57:00,388 --> 00:57:02,460 PROFESSOR: You put your foot in it. 990 00:57:02,460 --> 00:57:06,510 And that's why cowboy boots have a nice big heel, 991 00:57:06,510 --> 00:57:08,950 so your foot doesn't go all the way through it. 992 00:57:08,950 --> 00:57:09,920 It sticks in your heel. 993 00:57:11,590 --> 00:57:13,270 So this is the stirrup. 994 00:57:13,270 --> 00:57:15,850 You put your cowboy boot right in there 995 00:57:15,850 --> 00:57:18,130 until your heel hits this foot plate. 996 00:57:25,740 --> 00:57:28,740 That's pretty obvious how that got its name. 997 00:57:28,740 --> 00:57:31,095 It's the foot plate where you put your foot. 998 00:57:31,095 --> 00:57:32,360 Your foot goes right on that. 999 00:57:33,690 --> 00:57:35,980 And that foot plate is the beginning 1000 00:57:35,980 --> 00:57:39,750 of the next division, which is the inner ear. 1001 00:57:41,240 --> 00:57:43,510 And by the way, I should point out 1002 00:57:43,510 --> 00:57:47,935 before I forget-- what is the smallest bone in the body? 1003 00:57:50,070 --> 00:57:51,260 All answers are given. 1004 00:57:53,140 --> 00:57:55,840 The stapes is the smallest bone in the body. 1005 00:57:55,840 --> 00:57:56,632 Why? 1006 00:57:56,632 --> 00:57:57,340 It's got to move. 1007 00:57:58,410 --> 00:58:01,380 And the lousy little sound-- it's this tiny little ear drum. 1008 00:58:01,380 --> 00:58:04,090 Remember, the ear drum is basically 1009 00:58:04,090 --> 00:58:06,940 a tiny, little thin piece of skin. 1010 00:58:06,940 --> 00:58:09,080 It's like Saran wrap. 1011 00:58:09,080 --> 00:58:13,640 When your doctor looks down your ear canal, 1012 00:58:13,640 --> 00:58:16,500 that doctor can look right through the ear drum. 1013 00:58:16,500 --> 00:58:17,990 It's so thin. 1014 00:58:17,990 --> 00:58:19,870 It's like plastic wrap. 1015 00:58:19,870 --> 00:58:23,930 The doctor can look into the middle ear and say, 1016 00:58:23,930 --> 00:58:25,670 so much fluid in there. 1017 00:58:25,670 --> 00:58:27,700 You've got a middle ear infection. 1018 00:58:27,700 --> 00:58:30,251 Or they can say, middle ear looks good. 1019 00:58:30,251 --> 00:58:31,500 You've got some other problem. 1020 00:58:32,592 --> 00:58:33,800 That's what we're looking at. 1021 00:58:33,800 --> 00:58:36,930 They're looking with their otoscope and a light right 1022 00:58:36,930 --> 00:58:39,130 through the ear drum into the middle ear. 1023 00:58:39,130 --> 00:58:43,380 And that whole middle ear drum and the ossicles 1024 00:58:43,380 --> 00:58:47,985 have to vibrate when there's a tiny little sound like a pin 1025 00:58:47,985 --> 00:58:48,485 drop. 1026 00:58:50,160 --> 00:58:52,710 The pin drops right there, and you can hear it 1027 00:58:52,710 --> 00:58:55,890 because these things are so light and flexible that they 1028 00:58:55,890 --> 00:58:57,095 can vibrate-- and so small. 1029 00:59:01,360 --> 00:59:05,550 The stapes foot plate ends up at the cochlear. 1030 00:59:06,640 --> 00:59:11,530 And the cochlear is the main part of the inner ear. 1031 00:59:12,770 --> 00:59:16,670 And cochlear, as it says here, gets its name 1032 00:59:16,670 --> 00:59:20,090 from the Greek word kochlias, which means snail. 1033 00:59:21,970 --> 00:59:25,870 And certainly, the inner ear looks like a snail shell. 1034 00:59:28,170 --> 00:59:30,710 And in the inner ear, here's where 1035 00:59:30,710 --> 00:59:35,675 sound changes from sound in air, or maybe sound in the bones. 1036 00:59:37,030 --> 00:59:39,605 The inner ear is filled with fluid. 1037 00:59:58,200 --> 01:00:01,840 And inside the inner ear are these wonderful receptor cells 1038 01:00:01,840 --> 01:00:05,304 for hearing and the beginning of the auditory nerve. 1039 01:00:05,304 --> 01:00:06,970 Here's the auditory nerve that's sending 1040 01:00:06,970 --> 01:00:09,110 messages centrally into the brain. 1041 01:00:09,110 --> 01:00:10,980 So the brain would be beginning right here. 1042 01:00:13,460 --> 01:00:16,550 This whole structure here, all this gray stuff, 1043 01:00:16,550 --> 01:00:19,223 and even the shell of the cochlear is bone. 1044 01:00:19,223 --> 01:00:21,800 And it's your temporal bone. 1045 01:00:21,800 --> 01:00:25,280 The temporal bone is the hardest bone in the body. 1046 01:00:25,280 --> 01:00:28,890 You can have a severe blow to the head 1047 01:00:28,890 --> 01:00:31,580 and that temporal bone will keep all these structures intact. 1048 01:00:33,680 --> 01:00:34,920 It's very, very hard bone. 1049 01:00:36,220 --> 01:00:38,200 Surgeons at our hospital do a lot 1050 01:00:38,200 --> 01:00:39,757 of drilling with the dental drill. 1051 01:00:39,757 --> 01:00:41,590 They get down to these important structures, 1052 01:00:41,590 --> 01:00:44,350 because they have to manipulate them. 1053 01:00:44,350 --> 01:00:47,400 These loops here are part of the inner ear, 1054 01:00:47,400 --> 01:00:51,060 but they are part that is sensitive to vestibular 1055 01:00:51,060 --> 01:00:51,560 sensation. 1056 01:00:52,840 --> 01:00:56,070 So those loops are called the semicircular canals. 1057 01:00:56,070 --> 01:00:57,160 They are almost circular. 1058 01:00:57,160 --> 01:01:00,670 They are in the three planes, X, Y, and Z. 1059 01:01:00,670 --> 01:01:05,850 And when you rotate your head, let's say, side to side, 1060 01:01:05,850 --> 01:01:09,500 one of those can move. 1061 01:01:11,190 --> 01:01:15,930 And the receptor cells in it can sense that movement 1062 01:01:15,930 --> 01:01:18,450 and detect that your head had moved. 1063 01:01:18,450 --> 01:01:20,430 And it's very important, because if you 1064 01:01:20,430 --> 01:01:22,900 want to keep your eyes fixated on one point 1065 01:01:22,900 --> 01:01:25,200 but move your head, you can do that 1066 01:01:25,200 --> 01:01:28,615 by the vestibulo-ocular reflex. 1067 01:01:29,790 --> 01:01:32,610 The neurons from this the vestibular system 1068 01:01:32,610 --> 01:01:34,770 send messages into the brain stem, 1069 01:01:34,770 --> 01:01:37,450 and eventually they go through coordinating centers 1070 01:01:37,450 --> 01:01:42,070 into the motor neurons for the extraocular muscles, which 1071 01:01:42,070 --> 01:01:45,030 can, of course, move your eyes when you want 1072 01:01:45,030 --> 01:01:48,710 to do a [? secade ?] or pursuit, or they 1073 01:01:48,710 --> 01:01:52,050 can keep your eyes stabilized, which is moving them 1074 01:01:52,050 --> 01:01:55,090 with respect your head even though your head is moving. 1075 01:01:55,090 --> 01:01:57,372 But we're not going to talk about those. 1076 01:02:05,390 --> 01:02:08,330 Let's talk about the function of the middle ear 1077 01:02:08,330 --> 01:02:09,400 and the external ear. 1078 01:02:09,400 --> 01:02:11,899 That's what we're going to talk about for the rest of today. 1079 01:02:11,899 --> 01:02:12,892 I have a model. 1080 01:02:12,892 --> 01:02:14,350 Let me just pass around this model. 1081 01:02:14,350 --> 01:02:18,200 I think we passed around before on the first day of class, 1082 01:02:18,200 --> 01:02:21,140 but you can look at it again, because we're 1083 01:02:21,140 --> 01:02:23,850 going into more detail today on this structure. 1084 01:02:23,850 --> 01:02:25,630 This comes apart. 1085 01:02:25,630 --> 01:02:26,890 Here's your pinna. 1086 01:02:26,890 --> 01:02:28,045 Here's the long ear canal. 1087 01:02:30,540 --> 01:02:31,820 Here's the ear drum. 1088 01:02:33,460 --> 01:02:38,670 And if I tilt this here, you can see the structures 1089 01:02:38,670 --> 01:02:42,550 we're talking about in the inner ear-- the cochlear, 1090 01:02:42,550 --> 01:02:46,770 the semicircular canals, and this yellow structure here 1091 01:02:46,770 --> 01:02:47,925 is the auditory nerve. 1092 01:02:49,460 --> 01:02:51,370 It's been going into the brain. 1093 01:02:51,370 --> 01:02:52,700 The brain is cut off here. 1094 01:02:54,670 --> 01:02:57,790 This is the eustachian tube, which 1095 01:02:57,790 --> 01:03:04,220 is a way to vent the air-filled middle ear. 1096 01:03:04,220 --> 01:03:06,200 So you want to purge that with air. 1097 01:03:06,200 --> 01:03:08,850 If you go up hiking in a tall mountain, 1098 01:03:08,850 --> 01:03:12,030 the barometric pressure outside gets lower. 1099 01:03:12,030 --> 01:03:13,950 You want to equalize that in your middle ear. 1100 01:03:13,950 --> 01:03:16,625 You open that eustachian tube, usually by swallowing. 1101 01:03:18,880 --> 01:03:20,680 The ossicles are here. 1102 01:03:20,680 --> 01:03:23,090 And if you take out this inner ear, 1103 01:03:23,090 --> 01:03:26,000 the stapes is fixed with it. 1104 01:03:26,000 --> 01:03:27,120 So you can see the stapes. 1105 01:03:28,460 --> 01:03:33,860 In terms of size, this whole inner ear-- the cochlear 1106 01:03:33,860 --> 01:03:38,370 is about the size of an aspirin tablet in a human. 1107 01:03:38,370 --> 01:03:39,500 It's about that size. 1108 01:03:40,916 --> 01:03:41,416 OK. 1109 01:03:41,416 --> 01:03:42,853 Let's pass that around. 1110 01:03:48,981 --> 01:03:49,480 OK. 1111 01:03:49,480 --> 01:03:51,390 What is the function of the middle ear? 1112 01:03:52,610 --> 01:03:54,966 Why do we have these three bones? 1113 01:03:54,966 --> 01:03:56,090 Why do we have the eardrum? 1114 01:03:56,090 --> 01:04:00,321 Why doesn't sound come right in and strike the inner ear 1115 01:04:00,321 --> 01:04:00,820 itself? 1116 01:04:02,040 --> 01:04:05,520 Well, it turns out that if you look 1117 01:04:05,520 --> 01:04:13,590 at the physical characteristics of sound in air, 1118 01:04:13,590 --> 01:04:17,100 and you want to get that airborne sound 1119 01:04:17,100 --> 01:04:19,900 to sound in water, different medium. 1120 01:04:19,900 --> 01:04:21,375 So this is fluid or water. 1121 01:04:24,370 --> 01:04:25,125 This is air. 1122 01:04:27,179 --> 01:04:28,720 Sound is coming in here, and you want 1123 01:04:28,720 --> 01:04:33,490 to get it into the fluid of the inner ear, which is essentially 1124 01:04:33,490 --> 01:04:33,990 water. 1125 01:04:36,020 --> 01:04:40,430 If you don't do anything and you have the sound coming in here, 1126 01:04:40,430 --> 01:04:42,440 most of it bounces back off. 1127 01:04:43,760 --> 01:04:52,320 In fact, 99.5% of the energy of sound 1128 01:04:52,320 --> 01:04:57,020 in air at a fluid boundary is reflected back into the air. 1129 01:04:58,340 --> 01:05:04,210 So if you're in a boat here-- I didn't 1130 01:05:04,210 --> 01:05:08,700 draw this right-- you're in a boat here, you're fishing, 1131 01:05:08,700 --> 01:05:10,980 you're talking to your buddy in the back of the boat 1132 01:05:10,980 --> 01:05:14,950 and you say, pass me another beer, and your buddy says, 1133 01:05:14,950 --> 01:05:19,420 be quiet, you'll scare the fish-- actually, 1134 01:05:19,420 --> 01:05:20,650 the fish can't hear you. 1135 01:05:20,650 --> 01:05:27,230 Because most of the energy in your saying "pass me a beer" 1136 01:05:27,230 --> 01:05:29,200 bounced right back off into the air. 1137 01:05:31,380 --> 01:05:35,910 So how does the auditory system deal with this? 1138 01:05:35,910 --> 01:05:41,690 We want to listen very carefully to a pin drop, 1139 01:05:41,690 --> 01:05:45,030 but most of the energy bounces back off 1140 01:05:45,030 --> 01:05:48,760 at this boundary between air and fluid. 1141 01:05:49,830 --> 01:05:51,320 That's the job of the middle ear. 1142 01:05:57,710 --> 01:05:59,630 Here is how the middle ear moves. 1143 01:05:59,630 --> 01:06:02,680 This is a nice movie made by Heidi Nakajima 1144 01:06:02,680 --> 01:06:04,700 at Mass Eye and Ear Infirmary. 1145 01:06:04,700 --> 01:06:06,500 This orientation is a little bit different, 1146 01:06:06,500 --> 01:06:07,740 but this is the ear drum. 1147 01:06:08,780 --> 01:06:13,080 This is the malleus, the incus, and the stapes. 1148 01:06:14,250 --> 01:06:15,790 Together, they're the middle ear. 1149 01:06:17,810 --> 01:06:20,940 I said this inner ear is the cochlear here, 1150 01:06:20,940 --> 01:06:25,490 and it's encased in bone-- fluid encased in bone. 1151 01:06:25,490 --> 01:06:28,710 So how does this stapes work? 1152 01:06:28,710 --> 01:06:31,240 Well, there's a little window in the bone. 1153 01:06:32,380 --> 01:06:34,170 It's called the oval window. 1154 01:06:36,700 --> 01:06:44,816 And the foot plate of the stapes pushes on that oval window. 1155 01:06:44,816 --> 01:06:46,190 It's not indicated here, but it's 1156 01:06:46,190 --> 01:06:47,665 right underneath this oval part. 1157 01:06:48,970 --> 01:06:51,940 There's another window called the round window. 1158 01:06:53,160 --> 01:06:54,820 That's indicated by blue there. 1159 01:06:59,420 --> 01:07:01,850 And it's just a pressure relief point, 1160 01:07:01,850 --> 01:07:06,490 because if you pushed on fluid, it would push back to you. 1161 01:07:06,490 --> 01:07:08,193 Fluid is relatively incompressible. 1162 01:07:09,730 --> 01:07:14,070 So this pushing in means this membrane over the round window 1163 01:07:14,070 --> 01:07:15,700 can push out easily. 1164 01:07:15,700 --> 01:07:18,240 So it's easy to push in and pull back, 1165 01:07:18,240 --> 01:07:19,720 because this membrane can give. 1166 01:07:21,000 --> 01:07:24,740 As you can see, the motion of these bones 1167 01:07:24,740 --> 01:07:27,660 is coming into the fluid quite nicely 1168 01:07:27,660 --> 01:07:30,990 and changing some membranes inside the inner ear. 1169 01:07:32,150 --> 01:07:37,470 The job of the middle ear is so that most of that sound energy 1170 01:07:37,470 --> 01:07:43,430 gets into the fluids of the water of the inner ear. 1171 01:07:43,430 --> 01:07:45,020 How does it do that? 1172 01:07:45,020 --> 01:07:48,930 The primary way is by changing area. 1173 01:07:48,930 --> 01:07:56,520 The eardrum is this big drum, and the stapes foot plate 1174 01:07:56,520 --> 01:08:00,630 is this much lower in area or smaller structure. 1175 01:08:02,750 --> 01:08:04,455 And there's some formulas here. 1176 01:08:05,750 --> 01:08:12,920 p1, a1, those pressure and area at the tympanic membrane, 1177 01:08:12,920 --> 01:08:18,410 equals p2 a2 where the same characteristics at the stapes 1178 01:08:18,410 --> 01:08:19,149 foot plate. 1179 01:08:21,350 --> 01:08:27,569 So when you decrease the area a lot, a2 goes way down. 1180 01:08:27,569 --> 01:08:29,560 p2 has to go way up. 1181 01:08:31,260 --> 01:08:36,080 So that's then the main way that the middle ear 1182 01:08:36,080 --> 01:08:41,270 allows sound and air to go into sound and fluid. 1183 01:08:41,270 --> 01:08:45,916 The engineers would call this impedance matching. 1184 01:08:54,350 --> 01:08:57,810 And they would say that when you change media, 1185 01:08:57,810 --> 01:09:01,720 the impedance of one medium being different from another 1186 01:09:01,720 --> 01:09:04,479 means that most of the energy is going to bounce back off. 1187 01:09:04,479 --> 01:09:09,130 If you have a device here like the middle ear 1188 01:09:09,130 --> 01:09:14,040 to make the impedances more matching, much of this energy 1189 01:09:14,040 --> 01:09:16,689 is going to then go through the boundary from one 1190 01:09:16,689 --> 01:09:19,910 medium to the other if you match the impedances. 1191 01:09:19,910 --> 01:09:21,770 And one way of matching the impedances 1192 01:09:21,770 --> 01:09:23,910 is to change the areas. 1193 01:09:25,189 --> 01:09:29,270 Another way-- and this may be the reason we have three 1194 01:09:29,270 --> 01:09:33,989 and not just one middle ear bone-- is by a lever action. 1195 01:09:35,410 --> 01:09:38,819 So this is kind of like a lever where the fulcrum is off 1196 01:09:38,819 --> 01:09:41,420 to one side, not right in the middle. 1197 01:09:41,420 --> 01:09:43,990 And you can obviously get force amplification 1198 01:09:43,990 --> 01:09:45,405 from a lever action. 1199 01:09:47,210 --> 01:09:50,470 A third mechanism might be a buckling 1200 01:09:50,470 --> 01:09:51,795 of the tympanic membrane. 1201 01:09:52,960 --> 01:09:56,090 And you'll have to read-- I'm not an expert on that at all. 1202 01:09:56,090 --> 01:09:59,750 I'm not even sure if that's even in vogue these days. 1203 01:09:59,750 --> 01:10:03,970 But these actions are much less than the change 1204 01:10:03,970 --> 01:10:06,830 in area offered by the eardrum. 1205 01:10:08,610 --> 01:10:13,570 So what happens when a patient comes into the Massachusetts 1206 01:10:13,570 --> 01:10:16,720 Eye and Ear Infirmary, and for some reason, 1207 01:10:16,720 --> 01:10:22,710 either via an accident or a developmental problem, 1208 01:10:22,710 --> 01:10:24,240 they don't have an eardrum, and they 1209 01:10:24,240 --> 01:10:26,640 don't have these three ossicles. 1210 01:10:26,640 --> 01:10:30,940 So the sound goes right in from the outside 1211 01:10:30,940 --> 01:10:34,720 and strikes, let's say, the round window of the cochlear. 1212 01:10:34,720 --> 01:10:36,120 Are they deaf? 1213 01:10:36,120 --> 01:10:36,650 Well, no. 1214 01:10:36,650 --> 01:10:38,080 They have a hearing loss. 1215 01:10:38,080 --> 01:10:41,400 Some of the energy gets through into the fluid. 1216 01:10:43,410 --> 01:10:45,060 How big is their hearing loss? 1217 01:10:45,060 --> 01:10:48,290 Well, this is the so-called audiogram 1218 01:10:48,290 --> 01:10:52,450 that's generated when you visit a hospital 1219 01:10:52,450 --> 01:10:54,710 and you complain that your hearing isn't so good. 1220 01:10:54,710 --> 01:10:57,470 They send you down to the Audiology Department. 1221 01:10:57,470 --> 01:10:59,720 They put you in a testing booth. 1222 01:10:59,720 --> 01:11:04,280 They put earphones on, and the tester goes outside 1223 01:11:04,280 --> 01:11:06,590 so they don't make any extraneous noise. 1224 01:11:07,820 --> 01:11:13,270 And they say, raise your hand when you can hear a sound. 1225 01:11:13,270 --> 01:11:16,810 So they test your hearing-- this is the so-called audiogram-- 1226 01:11:16,810 --> 01:11:20,160 and plot it on the y-axis as the amount 1227 01:11:20,160 --> 01:11:23,300 of hearing loss in decibels. 1228 01:11:25,150 --> 01:11:27,150 It's just the way they plot it. 1229 01:11:27,150 --> 01:11:30,790 And plot it on the x-axis as the frequency. 1230 01:11:30,790 --> 01:11:35,720 And they typically test 2,550, 1,000-- 1231 01:11:35,720 --> 01:11:40,350 which is abbreviated here 1k-- 2k, and 4k. 1232 01:11:40,350 --> 01:11:44,850 They typically don't test the extremes of the human hearing. 1233 01:11:44,850 --> 01:11:46,280 They test the middle range. 1234 01:11:46,280 --> 01:11:51,630 This is the range over which most speech sounds are made. 1235 01:11:51,630 --> 01:11:54,250 And that's the most important for most people. 1236 01:11:54,250 --> 01:11:56,200 When they say, I can't hear very well, 1237 01:11:56,200 --> 01:11:58,890 it means they can't understand somebody when they're speaking. 1238 01:12:00,010 --> 01:12:02,530 And this is the audiogram from someone 1239 01:12:02,530 --> 01:12:04,620 who lacked a middle ear. 1240 01:12:04,620 --> 01:12:09,150 And this 40 dB here-- across all the different frequencies, 1241 01:12:09,150 --> 01:12:11,860 approximately 40 db-- is the amount of hearing loss 1242 01:12:11,860 --> 01:12:12,360 they have. 1243 01:12:13,630 --> 01:12:15,680 So if you go back to the audiogram 1244 01:12:15,680 --> 01:12:18,370 that we had in the first slide of today's lecture, 1245 01:12:18,370 --> 01:12:21,270 everything would be lifted up by 40 dB. 1246 01:12:21,270 --> 01:12:23,500 You have a 40 dB hearing loss. 1247 01:12:23,500 --> 01:12:26,420 You're not deaf at all, but that's 1248 01:12:26,420 --> 01:12:30,670 a moderate to severe hearing loss, a 40 dB hearing loss. 1249 01:12:31,870 --> 01:12:35,500 You might have problems-- you certainly 1250 01:12:35,500 --> 01:12:38,000 would have problems hearing a pin drop. 1251 01:12:38,000 --> 01:12:42,400 You might have problems hearing a telephone ring 1252 01:12:42,400 --> 01:12:44,500 if it were on the other side of the room. 1253 01:12:46,180 --> 01:12:48,563 You might have problems with conversation. 1254 01:12:50,580 --> 01:12:54,390 A treatment to that would be several types of treatment. 1255 01:12:54,390 --> 01:12:57,580 The surgeons in the Ear, Nose, and Throat Department 1256 01:12:57,580 --> 01:13:01,200 at Mass Eye and Ear could reconstruct 1257 01:13:01,200 --> 01:13:03,820 your middle ear and your eardrum. 1258 01:13:03,820 --> 01:13:06,510 They could use a skin flap, a piece of skin taken 1259 01:13:06,510 --> 01:13:08,490 from somewhere else on your body, 1260 01:13:08,490 --> 01:13:10,130 put it in the place of the eardrum. 1261 01:13:10,130 --> 01:13:15,200 They could use some either wire or Teflon or plastic pieces 1262 01:13:15,200 --> 01:13:20,510 that could connect that eardrum into the oval window 1263 01:13:20,510 --> 01:13:21,220 of the cochlear. 1264 01:13:21,220 --> 01:13:26,879 So they can reconstruct the middle ear fairly easily. 1265 01:13:26,879 --> 01:13:28,670 If the person doesn't want to have surgery, 1266 01:13:28,670 --> 01:13:31,310 they can have a hearing aid. 1267 01:13:31,310 --> 01:13:35,390 Essentially, you have a flat frequency loss here. 1268 01:13:35,390 --> 01:13:37,470 So put a device in the ear canal that 1269 01:13:37,470 --> 01:13:43,850 boosts every single frequency by 40 dB, amplify the sound. 1270 01:13:43,850 --> 01:13:47,000 So a hearing aid works pretty well 1271 01:13:47,000 --> 01:13:49,750 for these people with this type of a hearing loss. 1272 01:13:49,750 --> 01:14:00,490 This type of a hearing loss is called a conductive hearing 1273 01:14:00,490 --> 01:14:04,420 loss, because it's in the conductive mechanism 1274 01:14:04,420 --> 01:14:08,060 to conduct the sound from outside your body 1275 01:14:08,060 --> 01:14:09,350 into the inside of your body. 1276 01:14:09,350 --> 01:14:10,780 It's a conductive hearing loss. 1277 01:14:12,390 --> 01:14:14,645 So that is the job of the middle ear-- 1278 01:14:14,645 --> 01:14:18,150 to ensure efficient transmission of sound 1279 01:14:18,150 --> 01:14:20,410 in air into the fluids of your body. 1280 01:14:20,410 --> 01:14:22,881 And without it, you have a moderate to severe hearing 1281 01:14:22,881 --> 01:14:23,380 loss. 1282 01:14:26,250 --> 01:14:28,290 There's a disease called otosclerosis. 1283 01:14:45,170 --> 01:14:46,752 "Oto" meaning hearing. 1284 01:14:50,270 --> 01:14:52,490 My department at Harvard Med School 1285 01:14:52,490 --> 01:14:54,520 is otology and laryngology. 1286 01:14:55,690 --> 01:14:57,310 Otology and laryngology. 1287 01:14:58,920 --> 01:15:03,550 And sclerosis means hardening or rocky or bony growths. 1288 01:15:09,060 --> 01:15:13,980 And the surgery that happens-- sometimes 1289 01:15:13,980 --> 01:15:18,170 around the stapes, bony growths can grow around it 1290 01:15:18,170 --> 01:15:21,480 and fix the foot plate so that it can't vibrate anymore. 1291 01:15:23,480 --> 01:15:26,720 So what's done for that is you take out the stapes, 1292 01:15:26,720 --> 01:15:29,710 you take off the bony growths, and if you just 1293 01:15:29,710 --> 01:15:33,760 put the stapes back in, often these bony growths grow again. 1294 01:15:33,760 --> 01:15:35,910 So actually you take it out and replace it 1295 01:15:35,910 --> 01:15:37,780 with an artificial stapes. 1296 01:15:37,780 --> 01:15:40,015 And the operation is called a stapedectomy. 1297 01:15:46,010 --> 01:15:49,230 The "stape" and "ectomy" means taking it out. 1298 01:15:49,230 --> 01:15:51,770 You replace it with a prosthesis. 1299 01:15:51,770 --> 01:15:56,020 It's a very successful surgery for otosclerosis, 1300 01:15:56,020 --> 01:15:57,700 which is a conductive hearing loss. 1301 01:15:59,950 --> 01:16:01,560 That's the job of the middle ear, 1302 01:16:01,560 --> 01:16:07,190 and that's relatively easy to treat when there's a problem. 1303 01:16:09,300 --> 01:16:12,110 Is there a function of the external ear? 1304 01:16:13,980 --> 01:16:17,980 Well, a lot of textbooks say the external ear funnels the sound 1305 01:16:17,980 --> 01:16:19,075 into your ear canal. 1306 01:16:21,000 --> 01:16:26,320 But there is another function of the external ear that's 1307 01:16:26,320 --> 01:16:32,050 more on the lines of localizing sounds using your external ear. 1308 01:16:34,410 --> 01:16:36,920 These are examples of external ears-- our pinna. 1309 01:16:38,400 --> 01:16:41,070 Everybody has a slightly different one. 1310 01:16:41,070 --> 01:16:42,688 Who is this historical figure? 1311 01:16:42,688 --> 01:16:43,188 Anybody? 1312 01:16:49,180 --> 01:16:51,490 He was a president of the United States. 1313 01:16:51,490 --> 01:16:54,600 LBJ, President Johnson. 1314 01:16:54,600 --> 01:16:59,220 He was always caricatured by the political cartoon guys 1315 01:16:59,220 --> 01:17:02,707 with these huge ears, big pinnae. 1316 01:17:02,707 --> 01:17:04,290 Everybody has different shaped pinnae. 1317 01:17:06,940 --> 01:17:10,820 It turns out that the external ear can help you localize 1318 01:17:10,820 --> 01:17:12,140 where sound is coming from. 1319 01:17:14,590 --> 01:17:15,895 Well, how can it do that? 1320 01:17:19,030 --> 01:17:20,930 Well, if you have a pinna and you 1321 01:17:20,930 --> 01:17:36,270 do this interesting experiment-- you take a microphone 1322 01:17:36,270 --> 01:17:38,820 and put the microphone inside here. 1323 01:17:38,820 --> 01:17:39,795 So here's the pinna. 1324 01:17:41,200 --> 01:17:42,290 Here's the ear canal. 1325 01:17:43,500 --> 01:17:46,200 Put the microphone out here, and start out 1326 01:17:46,200 --> 01:17:50,565 with a completely flat spectrum, broadband noise. 1327 01:17:52,910 --> 01:17:55,810 The noise is absolutely flat so that it 1328 01:17:55,810 --> 01:17:58,040 has equal energy at all the frequencies. 1329 01:17:59,720 --> 01:18:03,110 You measure it out there, and then you move your microphone 1330 01:18:03,110 --> 01:18:09,130 down here in the ear canal, maybe near the eardrum , 1331 01:18:09,130 --> 01:18:12,120 and measure the spectrum again. 1332 01:18:13,500 --> 01:18:16,830 So this is plotted in terms of gain 1333 01:18:16,830 --> 01:18:19,250 with respect to free field. 1334 01:18:19,250 --> 01:18:20,540 Free field is out here. 1335 01:18:25,840 --> 01:18:28,860 Free field means basically in the room or in the environment. 1336 01:18:29,970 --> 01:18:32,000 Now we're going to measure the spectrum down 1337 01:18:32,000 --> 01:18:35,150 here and plot the gain. 1338 01:18:35,150 --> 01:18:39,080 So anything above 0 is going to be higher than in the ear, 1339 01:18:39,080 --> 01:18:41,270 and everything below 0 is going to be lower. 1340 01:18:43,750 --> 01:18:47,450 Let's look at this solid curve here, 1341 01:18:47,450 --> 01:18:50,690 which is minus 15 degrees elevation. 1342 01:18:52,200 --> 01:18:56,590 Elevation of a sound source-- if it's straight ahead, it's zero. 1343 01:18:56,590 --> 01:18:59,725 If it's minus 15, it's 15 degrees below zero. 1344 01:19:01,040 --> 01:19:04,510 If it's above zero, it could be 15 degrees. 1345 01:19:04,510 --> 01:19:06,730 On this case, it's 7.5 and 30. 1346 01:19:06,730 --> 01:19:09,360 So elevations that are positive are above you. 1347 01:19:11,280 --> 01:19:13,640 As that sound source moves around 1348 01:19:13,640 --> 01:19:19,630 from being below you to above you, its spectrum changes, 1349 01:19:19,630 --> 01:19:21,780 the spectrum way down here at the ear drum. 1350 01:19:22,980 --> 01:19:27,200 And in particular, there are some very sharp dips 1351 01:19:27,200 --> 01:19:29,705 or nulls in the spectrum that move around. 1352 01:19:30,760 --> 01:19:35,860 It's thought that you can use those nulls as a cue 1353 01:19:35,860 --> 01:19:37,530 to where this sound is. 1354 01:19:39,810 --> 01:19:41,870 Now, what causes those nulls? 1355 01:19:41,870 --> 01:19:46,390 Well, because the pinna is very complicated, 1356 01:19:46,390 --> 01:19:51,640 you can imagine that some sound comes in and strikes the pinna 1357 01:19:51,640 --> 01:19:53,490 and reflects off it. 1358 01:19:53,490 --> 01:19:56,960 And maybe it reflects-- excuse my artistic abilities 1359 01:19:56,960 --> 01:20:00,770 here-- maybe it reflects into the ear canal. 1360 01:20:00,770 --> 01:20:05,260 Contrast that with other sound that comes straight in. 1361 01:20:06,830 --> 01:20:10,640 Eventually, these two sounds are going to meet up at a point. 1362 01:20:10,640 --> 01:20:15,650 And let's say this sound taking a longer time path 1363 01:20:15,650 --> 01:20:17,780 went through half of its cycle. 1364 01:20:18,790 --> 01:20:22,230 So now this sound, when it's starting 1365 01:20:22,230 --> 01:20:25,360 to go a negative pressure, meets up 1366 01:20:25,360 --> 01:20:28,290 with this sound, which came straight in and is starting 1367 01:20:28,290 --> 01:20:30,450 to go in a positive pressure. 1368 01:20:30,450 --> 01:20:33,165 Positive plus negative could sum to zero. 1369 01:20:34,730 --> 01:20:36,990 And the geometry has to be just right, 1370 01:20:36,990 --> 01:20:39,540 and the frequency has to be just right. 1371 01:20:39,540 --> 01:20:42,700 But it can be just right at a particular frequency, 1372 01:20:42,700 --> 01:20:44,230 and that's what causes the nulls. 1373 01:20:44,230 --> 01:20:46,480 It's just a physical characteristic 1374 01:20:46,480 --> 01:20:49,160 of two sound sources meeting up. 1375 01:20:49,160 --> 01:20:51,580 It is thought, then, that you can 1376 01:20:51,580 --> 01:20:55,060 learn the position of those nulls, 1377 01:20:55,060 --> 01:20:59,830 especially, to be associated with positions of sound 1378 01:20:59,830 --> 01:21:00,690 in space. 1379 01:21:00,690 --> 01:21:04,399 And that's what was done in the researchers' report for today. 1380 01:21:09,450 --> 01:21:11,685 These are some data from four different subjects. 1381 01:21:14,030 --> 01:21:17,230 They tested the subject to localize 1382 01:21:17,230 --> 01:21:20,180 sounds coming from in front of them. 1383 01:21:20,180 --> 01:21:21,570 Left and right would be azimuth. 1384 01:21:23,070 --> 01:21:24,350 That's plotted on the x-axis. 1385 01:21:26,450 --> 01:21:28,728 Up and down would be elevation. 1386 01:21:28,728 --> 01:21:29,936 That's plotted on the y-axis. 1387 01:21:31,950 --> 01:21:35,660 And they move to sounds around to different places, 1388 01:21:35,660 --> 01:21:38,150 and they said to the person, tell me 1389 01:21:38,150 --> 01:21:39,910 where the sound is coming from. 1390 01:21:42,600 --> 01:21:44,720 The answers that the subjects gave 1391 01:21:44,720 --> 01:21:47,680 are in these solid, thick lines. 1392 01:21:47,680 --> 01:21:50,180 The real positions were on the thinner lines. 1393 01:21:51,200 --> 01:21:55,830 And each big individual data points are the small points, 1394 01:21:55,830 --> 01:21:57,990 and the average data points from the subject 1395 01:21:57,990 --> 01:21:59,370 are the big points here. 1396 01:22:00,620 --> 01:22:05,880 So these subjects, when given a checkerboard of locations, 1397 01:22:05,880 --> 01:22:11,210 they could pretty faithfully tell the investigators 1398 01:22:11,210 --> 01:22:13,990 where a sound was coming from, both in elevation 1399 01:22:13,990 --> 01:22:14,770 and in azimuth. 1400 01:22:16,007 --> 01:22:17,840 These are data from four different subjects. 1401 01:22:19,330 --> 01:22:23,427 What was done in the experiment is distort the pinna. 1402 01:22:23,427 --> 01:22:24,510 How are we going to do it? 1403 01:22:24,510 --> 01:22:27,800 Well, we could move our ear a little bit. 1404 01:22:27,800 --> 01:22:30,620 What they did was they put in a little clay 1405 01:22:30,620 --> 01:22:33,997 mold in parts of the pinna to change the shape, 1406 01:22:33,997 --> 01:22:35,330 and they did that on both sides. 1407 01:22:37,290 --> 01:22:40,510 As soon as they did that, these are now 1408 01:22:40,510 --> 01:22:41,840 the answers from the subjects. 1409 01:22:43,740 --> 01:22:47,840 Terrible in terms of elevation sensitivity, 1410 01:22:47,840 --> 01:22:49,510 determining where a sound is coming 1411 01:22:49,510 --> 01:22:51,980 from in terms of different elevations. 1412 01:22:51,980 --> 01:22:53,350 Still pretty good in azimuth. 1413 01:22:54,890 --> 01:22:57,490 There are other queues for sound azimuth 1414 01:22:57,490 --> 01:23:00,250 that involve using two ears, which we're 1415 01:23:00,250 --> 01:23:02,490 going to talk about extensively later this semester. 1416 01:23:04,060 --> 01:23:06,670 The elevational localization was completely 1417 01:23:06,670 --> 01:23:10,170 disrupted when the pinna shape was disrupted. 1418 01:23:12,480 --> 01:23:15,540 Have these subjects go out for a few weeks, come back, get 1419 01:23:15,540 --> 01:23:16,380 tested again. 1420 01:23:17,420 --> 01:23:21,210 They re-learned with the pinna molds 1421 01:23:21,210 --> 01:23:24,905 in how to localize sounds. 1422 01:23:26,320 --> 01:23:29,340 This is an example of re-learning or plasticity. 1423 01:23:31,670 --> 01:23:35,030 Now the pinna cues had different nulls 1424 01:23:35,030 --> 01:23:36,900 because the pinnas were shaped differently. 1425 01:23:36,900 --> 01:23:38,940 They could re-learn these new cues 1426 01:23:38,940 --> 01:23:43,170 and associate them with the same old changes in elevation 1427 01:23:43,170 --> 01:23:44,470 that we had before. 1428 01:23:44,470 --> 01:23:47,650 So that's why it's called re-learning sound localization 1429 01:23:47,650 --> 01:23:50,970 with new ears or new or distorted ears. 1430 01:23:50,970 --> 01:23:56,790 So this is an example then, of subjects learning 1431 01:23:56,790 --> 01:24:01,240 to associate these new cues with the old sound localization 1432 01:24:01,240 --> 01:24:02,420 positions. 1433 01:24:02,420 --> 01:24:11,400 So that's the take home message from this research report 1434 01:24:11,400 --> 01:24:17,830 OK questions I can also do I get on Wednesday, 1435 01:24:17,830 --> 01:24:21,020 we'll talk about the inner ear.