14. Sound: External, middle and inner ears


The following
content is provided under a Creative
Commons license. Your support will help MIT
OpenCourseWare continue to offer high quality
educational resources for free. To make a donation or to
view additional materials from hundreds of MIT courses,
visit MIT OpenCourseWare at ocw.mit.edu. PROFESSOR: And I thought
that what we’d do today is first go over this
syllabus for audition, which is the second part
of the course. Just so you get an idea of
what’s in store for you. And then today’s lecture
will have a big part on sounds, which have
physical properties that are very different than the
light stimulus you guys have been talking about
so far in the course. And we’re going to illustrate
the different types of sounds, very simple sounds
like pure tones. And very complex sounds
like human speech, which have many, many
components in them. And then we’ll get into
the auditory system, first starting with
the auditory periphery. And we’ll talk about the
three basic divisions of the auditory periphery,
which are the outer, middle, and inner ear. And today we’re
really only going to have a chance to
focus on the functions of the outer and
the middle ears. And so we’ll talk about the
functions of those structures. So as far as the syllabus
goes, each of the lectures has a title. So today’s title, October 28th,
is sound, external, middle, and inner ears. And each of the lectures
has a reading or more accompanying it. And so most of the readings
are from the textbook. So there’s a textbook,
Schnupp, Nelken, and King, which is a very good,
up-to-date textbook written by two psychophysicists. And one physiologist,
Israel Nelken. And it’s written at just the
right level for this class. That is an advanced
undergraduate textbook. So it’s pretty easy to read,
or should be very easy to read. And it’s written very well. These guys are good writers. They have many examples
of auditory demonstrations that you can listen
to just by clicking in the margin of the text. The demonstration we’ll come up. And as you can see,
after today’s lectures, I like to give demonstrations. Because I like to
listen to what we’re talking about in
terms of how does it really sound to
you as a listener. So I’d encourage you
to get that textbook. Now you could buy a hard
copy, or if I’m not mistaken, Michelle you can get
a free copy online. Is that right? AUDIENCE: There’s
an online version as well that you can
read on [INAUDIBLE]. PROFESSOR: OK. Great. So if you have any trouble
figuring that out, let me know. But I think you should
easily be able to find the online version. And it should have
the demonstrations that you can listen to
with earbuds or headphones. There is for today these
passages from the textbook. And then for today and
for many of the lectures, there is another reading,
which is a research paper. This one is by Hofman,
Van Riswick, and Opstal. And it’s titled, Relearning
Sound Localization with New Ears. And this we’ll talk
about in class right at the end of
today’s lecture when we talk about the
function of the outer ear, the pinna, so-called. And they did a very
interesting experiment that addresses what is the
function of your external ear. So people always ask me,
what am I responsible for in these readings. Well this is a very
specific paper. It has a lot of interesting
research methods. The subjects were
human volunteers. And there are a lot
of details in there that are not that important. What I’m really focused
on having you learn is the take-home message. And the take-home
message from this paper is, what is the function
of the outer ear. And what is this twist
in the title, how can you relearn sound localization
with different outer ear. We’ll talk about it in class. But I want you to get
the take-home message from these research studies. Because there sort of what
we do as professionals in the auditory system. Our day-to-day living
is doing research. In some cases on human
subjects, in some cases on individual molecules. But how can we
learn about hearing from doing these
research studies? And I have picked good
papers, good research studies. Because they really
tell us something. There’s plenty of
stuff out there that gives sort of
equivocal results. But this is a really good paper. And you have a
take-home point from it about how you use your outer
ears to localize sounds. So that’s an example
of a research paper that goes along
with this lecture. So just coursing through
the syllabus– on Wednesday, we’ll have a lecture
on hair cells. Next week we’ll talk about
the auditory nerve, which is the nerve that sends hearing
information from your ear into your brain. And we’ll talk about
frequency resolution– how we can tell one
frequency from another. At the end of next
week, we’ll be talking about the brain, the
cochlear nucleus, and all the interesting unit and cell
types in the cochlear nucleus. The following week,
we’re going to be talking about hearing
loss, how there can be problems with your hearing. Many of them are treated
at the hospital where I do my research, which is
Massachusetts Eye and Ear Infirmary across the river. And there, when the
surgeons in my department encounter a deaf
person, they give them the option to get
a cochlear implant. So a cochlear
implant is a device that can be put
in your inner ear. And it can restore
your sense of hearing. And we’ll have a demonstration
by that cochlear implant user who comes to
class on that date and gives a demonstration of
her cochlear implant, which has restored hearing to
her, although not perfectly. So then later on
in the semester, we’ll talk about
various other topics on up through the
auditory cortex. And finally, we’re going to have
a tour of the Hearing Research Laboratory at the Massachusetts
Eye and Ear Infirmary, where we’ll meet
over there and we’ll encounter various
research projects that are currently going on. And we’ll talk about them. There is a written assignment. I guess you guys had an
assignment for vision in the class– a written paper? So we have an analog here
on the auditory system. And this is the assignment
you can read it later at your leisure. It won’t make much
sense right now, because we haven’t talked
about neural circuits for localization of sounds yet. You can look on the syllabus. It’s about halfway through
the second part of the class. And there’s a lot
of details here. And it asks you what’s updated
since an original model was postulated by a researcher
called [? Jeffrus ?]. So that’s a paper–
I don’t think I said how long it should be. How long was the
paper for vision? Was there a link? AUDIENCE: [INAUDIBLE] PROFESSOR: Four to six pages? OK, four pages sounds good. If you really want to write
six, you probably could. But we’ll talk
about this when we talk about sound
localization in the class. And I think the due
date here is written. It’s the date of the lab tour. And then, we have a
final exam in the class. And I think as Doctor
Schiller talked about at the very first
day, the final exam will be waited toward
the auditory system, which we haven’t had a test
on by the time this exam rolls around. So I think it’s going
to be 2/3 audition on the final exam,
and 1/3 vision. And there are several
review sessions for both senses planned at
the end of the semester. So any questions
about the organization of what we’re going to do? OK, so I’ll start
today’s lecture. And I think the PowerPoint
files– for today’s lecture and all the rest of the lectures
for the rest of the semester are available in
the course website. So you can look at them now
or as the lecture comes up. So first, we’re
going to talk about the physical
characteristics of sound– just very, very different
than the characteristics of the light stimulus. And maybe light stimuli are
so obvious that Peter Schiller probably didn’t spend much
time in his lecture about it. But I’m going to spend
10 or 15 minutes here on the physical
characteristics of sound. Because it’s very different
than the light stimulus. So sound is a mechanical,
radiated energy, transmitted by longitudinal
vibrations of a m so you have to have a
medium to transmit sound. You can have light go
through outer space in a complete vacuum. But in outer space,
you can’t have sound because you have
to have a medium. The medium can be
various types of things. We’re going to talk
mostly about sound in air. But you could have sound in
water– whales make songs, and they sing to each other, and
one whale listens to another. And in between the two
is a medium of water. You can have sound in a solid–
if you live in an apartment room, and you hear your
neighbors’ music, you especially here the base. Because the low frequency
sound transmits pretty well through solids, like
the solid of the wall in between the two apartments. Sound can go in many, many
different types of media. In air, like we’re going to
use mostly for this course, you can think of sound
is being produced by a sound source like the
piston of your loudspeaker. And the piston goes
back and forth. . It’s driven back and forth
by an electric voltage and when it goes this way, it
presses on the air molecules in front of it. And it presses them so
they’re closer together and makes them a little
bit higher in pressure. And that’s what’s meant by this
compression or condensation. And these dots close
together means a little bit of an area of high pressure. Then as the piston moves
the other direction, it rarefies the air. It drags some of
the air with it. And so that little space
right in front of the piston has a lower pressure. Because there are fewer
molecules per volume than before. So this energy, then, is
transmitted through the medium to whatever– a microphone,
which detects sound, or a listener, which
can listen to the sound. If you have a microphone or
some kind of detector that can plot that pressure
at any one point– let’s say the microphone
is right at the edge of this paper. And you graph the pressure as a
function of time on this graph. So here’s pressure
and here’s time. As those radiated energy
wave fronts pass you, the pressure will go up. And then it will go down. And then it will
go up and go down. And it will repeat over and over
as long as that piston moves. So this horizontal
line is simply the barometric or static
pressure of the air. And sure, the barometric
pressure changes a little bit. If there’s a hurricane
coming, it gets way low. If there’s a high pressure
like we have right now– sunny climate, the
barometric pressure goes up. But those are very
slow fluctuations. And the sound wave form is
a very, very fast waveform that goes many times per second. In fact, what we call
as sound frequency is the number of oscillations of
that pressure wave per second. And they are very fast. As you can see down here
on this so-called audiogram or frequency curve
for human hearing, the frequencies, which
are on the x-axis here, go from 10 Hertz– Hertz
means cycles per second– so one Hertz is one
cycle per second. And that is a
frequency that’s so low it didn’t get on this graph. Because humans
aren’t sensitive to frequencies that
slow or that low. Usually the lower
limit for human hearing is considered to be about 10
cycles per second, or 10 Hertz. And it extends all the way up
to 20,000 cycles per second. And in the middle
of the human range, we’ll be talking
about hearing a lot at a middle frequency
of about 1,000 Hertz. So that’s a nice, round,
middle frequency for you to remember for human hearing. So we’re talking about
pressure oscillations in terms of thousands
of times per second, or hundreds of times per second. So they’re very fast. There will be examples
during our course where the auditory system–
the auditory neurons keep track of those
cycles, even though they’re going back and forth
thousands of times per second. So we’ll come back to
that in future lectures. Now this is the audiogram
for human hearing in the solid curve here. This is supposed to say
human, if you could read it. And on the y-axis is
how strong the stimulus is, how loud it is, or in terms
of physical characteristics, what the sound pressure is. And this scale goes
from minus 20 to 140. And the units are
dB SPL and that stands for decibels
sound pressure level. And whenever you hear
level in a formula, you should perk up your
ears and say oh, that means there’s a log– a
logarithm– in the formula. And sure enough, the formula
for a sound pressure level is 20 times the log of
whatever sound pressure you’re talking
about, whatever you were listening to or measured
by your microphone divided by some reference pressure. That’s the formula. And the reference
pressure is given as 20 micronewtons
per square meter. OK, so let’s figure that out. What is Newton a unit of? Anybody? AUDIENCE: Force. PROFESSOR: Right, force–
and meter squared is area. So we’re talking about force
per area, and that’s pressure. So Newton obviously was like
Hertz, one of the people who was interested in physics. And a Newton is a unit of force
per square meter is pressure. Now in more modern terms,
the unit micronewton per square meter
has been renamed to be a pascal, abbreviated Pa. So it’s the same. One Pascal is one
Newton per square meter. In this case, we’re talking
about micro– Newtons are micro Pascals. So why is that number
chosen as the reference for this very important
sound pressure level scale? Well, it’s actually chosen with
the hearing system in mind. What they did in the 1930s,
when this was being developed, is they rounded up a bunch
of people at a county fair, gave them headphones,
and said we’re going to try a
nice mid-frequency. Let’s try 1,000 Hertz. They gave them a
tone at 1,000 Hertz. The listeners listened to it. Then they said I
can hear that fine. Then they turned the
level down a little bit. And the person said yeah,
I can still hear that. Then they turned it down so
much that the person didn’t say, I hear something. There were silent. They turned it up a
little– says yeah, I hear– They turned it down. They titrated the levels until
it was right at threshold, just barely detectable. And they took an average
of 30-some people. And they said that
is going to be the basis of our sound
pressure level scale. So it’s actually a term that
was derived biologically by testing people’s hearing. So that’s kind of a nice story. I wonder if it’s true. Well, let’s look at it. Where does the human hearing
curve, that 1,000 Hertz, fall? Where should it fall if 20
micronewtons per square meter is the pressure
you’re talking about? It’s the same as the
reference pressure. What’s 20 over 20? It’s 1. What’s the log of 1? Zero. Correct. 20 times the log of 1 is
0– sound pressure level 0. Well, look at our
curve right here, that 1,000 Hertz–
it’s pretty close to 0. Why might it not
be exactly zero? Well the people that
were used for this curve were a little bit different than
the ones in the county fair. We’ll study later on that some
people have a hearing loss. Hearing can be affected
by the room that you used. Maybe there was a lot
of yelling and screaming at the county fair. We have better rooms
to test hearing now. It turns out that the human
hearing curve is actually a little more sensitive at
2,000, 3,000, and maybe 4,000. So when the pressures go
below the reference pressure, the number becomes less than 1. And the logarithm
becomes negative. It’s perfectly fine to
have a negative SPL. We have some points on the graph
for that– minus 2, minus 3 dB. This other dashed audiogram,
or hearing sensitivity curve, is for a different
species– the cat. And the cat here’s
down to about minus 10 dB SPL– at least this
group of cats did. The cats also hear higher
in frequency than humans. Dogs and cats can
hear about an octave higher– that is a
doubling of frequency higher than humans do,
and maybe some of you have had dog whistles
that you blow. And you don’t hear anything. But the dog comes because
it’s a very high frequency beyond the upper limit
of human hearing, but well within the hearing
range of those species. So different species have
different hearing ranges. AUDIENCE: Professor? PROFESSOR: Yes. AUDIENCE: Sorry–
just to clarify, is a micropascal
then [INAUDIBLE]? PROFESSOR: No. These are units of pressure–
micronewtons per square meter– and this is a unit of pressure. SPL is just in these
units called decibels. And it it’s not a pressure– AUDIENCE: It’s the log of that. PROFESSOR: That’s right. It’s the log of that. Any other questions? So these are sort of the
lower limits of hearing. When you go into conversational
levels, or the level of a lawn mower, or the
level of a concert, the levels get higher–
still certainly within your audibility range. As you go to a higher
and higher level, you risk damage to your hearing. And at that risk level, which it
says high risk thresholds here. And right around 120 dB,
sounds become painfully loud and damaging
to your hearing. And that’s what this
shaded area refers to– gunshots, jet
aircraft engine. And we’ll talk about that during
our lecture of hearing loss. So I have some demonstrations. Because a lot of people have
trouble with the decibel scale. So what is a decibel? And what does it sound like when
you change the sound from 50 dB to 60 dB? Well this demonstration
has three parts. And let me read the text first. Broadband noise– sometimes
it’s called white noise. Broadband noise and
white noise are synonyms. And what is white light
as a visual stimulus? AUDIENCE: All wavelengths. PROFESSOR: All
wavelengths, right? And so broadband noise means
it has all frequencies. It has 10 Hertz, 20
Hertz, 30 Hertz, 1,000 Hertz, 2,000– it
has all frequencies. And it sounds like
the “shh” sound. So you hear this “shh.”
it’ll start out pretty loud. It’ll be reduced in ten steps
of six decibels for each step. And I think you’ll be
able to very clearly hear the difference between the
first and the second steps. And demonstrations
are repeated once. The second demonstration
is same noise is reduced in 15 steps–
now of three decibels. So this is a little
bit of a smaller scale, though you’ll still
be clearly audible. Third, broadband
noise is reduced in 20 steps of now one dB. So let’s listen to see if
we can hear 1 dB steps. RECORDING: The decibel
scale– broadband noise is reduced in
10 sets of 6 decibels. [INAUDIBLE] repeated once. [TONE] [TONE] OK, was that clear–
the difference between one and the other? So that’s what 6 dB sounds like? Now, you guys who are up
here close to the speakers, you might be starting at 85 dB
SPL on the first ones– pretty loud. 6 dB lower is 79. And then, so on and so forth. You guys at the back are
further from the speaker. You’re not starting
at the same level. You might be starting at 60 dB. You’re still going down 6 dB
to 54 dB in the next step. Everything is linear in here. It doesn’t matter
where you start from, as long as you’re
going down 6 dB. So where you start doesn’t
really matter in these demos. RECORDING: Broadbad
noise is reduced in 15 steps of 3 decibels. [TONE] [TONE] PROFESSOR: OK, still
clear the increment between one and the other? OK, now here’s the one dB steps. RECORDING: Broadband
noise is reduced in 20 steps of one decibel. [TONE] [TONE] PROFESSOR: OK so
how about for that? Would you be able to stake
your life on the fact that you could tell
one from another? No, I see a lot
of heads shaking. Well if you sit there and
do this over and over again, and really train
yourself, apparently 1 dB is the just
noticeable difference that most observers can here. So 1 dB is the just
noticeable difference in SPL. So how do we do that? Well you have an auditory nerve. And at 60 dB, your
auditory nerve fibers are sending this many
spikes to the brain. At 61 dB, they’re sending maybe
a few more spikes– something like that. It’s not absolutely
clear how you do that. There is more information coming
in from the ear to the brain as a function on sound level. We’ll talk a lot about that. Now, we also talked
about sound frequency. JND for sound level
is about 1 dB. What is it for sound frequency? We’re going to have pretty
much a whole lecture on that. But your ear is extremely
good at telling one frequency from another. So if you start at 1,000
Hertz and change it to 1,002 Hertz– very, very
small change– you can tell the difference. Your ear is a fantastic
frequency analyzer. We’re going to have
a whole lecture on exactly how
your ear does that. But the JND for sound frequency
is also a good demonstration. We’ll play that when we talk
about sound frequency coding. OK. Any questions about that so far? OK. Let’s switch back to the
physical characteristics of sound. And these are some very
common auditory stimuli. We’ve heard a noise just now. And if you graph the sound
pressure as a function of time, this is what the
waveform looks like. How could you do that? If you take a
microphone, stick it out in front of a noise source, and
run that into an oscilloscope, the microphone converts the
sound pressure into a voltage, the oscilloscope displays
the voltage signal as a function of time. You can look at that. Auditory scientists
like to look at things as a function of
time, of course. They also like to look
at things as a function of sound frequency. This is a graph for this same
stimulus, a noise stimulus, now as a function of frequency. And we said before,
the noise is broadband. It’s white noise. It has all frequencies. And here is the graph
to show you that. This might be the
energy, and this is as a function of frequency. So it has all frequencies. It’s trailing off a little
at the very highest. That may be because the
microphone couldn’t wiggle back and forth at very,
very high frequencies. But it’s essentially a
flat frequency curve. And sometimes this display
is called the spectrum. So spectrum or spectra
are graphs as a function of frequency. Sometimes people talk
about this as a frequency domain and the time domain. If you’ve taken any electrical
engineering courses here at MIT, people will talk about
the time and frequency domains. And how can you go from one
representation to another? Well, you can take
your microphone signal instead of going to
the oscilloscope, going to the spectrum
analyzer, which is a machine that can
give you this nice plot. But how about mathematically? How can you do that? The Fourier Transform, right. Of course, Fourier
was a mathematician who studied various things,
heat transfer and other things. He developed this
transformation. If you have the
mathematical description of a time-varying
signal, you can plug it through his equation,
the Fourier transform, and come out with the frequency
representation or the frequency domain. Or, vice versa, if you
have the frequency domain, you can inverse Fourier
transform and go back to the time domain. We’re not going to talk too
much about transforms here. But it is interesting,
because, as it turns out, your inner ear is a
wonderful frequency analyzer. It can tell the difference
between 1,000 and 1,002 Hertz. This is a very
nice way in the ear of detecting the
different frequencies. And so these time and frequency
domain representations are very convenient
for us to look at. So just keep that in mind. Here’s a very common auditory
stimulus, the pure tone or the sinusoid. This is a sinusoidal
waveform in the time domain. In the frequency domain,
it only has one frequency– the frequency at
which that thing is going back and forth
in terms of Hertz. This is in a Hertz axis. So sometimes it’s
called a pure tone. Why is it so pure? Does it have high
morals or what? No, it just has one
sound frequency. These other stimuli,
we’re going to listen to this in just a minute. This is a so-called square wave. Imagine trying to add up a
whole bunch of pure tones to result in a square wave. It seems impossible, right? Well, it’s possible if
you use an infinite number of frequencies. And this frequency
representation for a square wave goes
on basically forever. To get those corners
of the square wave sharp like a true
square wave, you need lots of
individual frequencies, lots of pure tones, if you will. Tone bursts are some
common auditory stimuli. We’ll talk about those
later in the course. Click is a very common
auditory stimulus. It’s a sound like this. Or last night, it was
the sound of a fastball hitting a wooden baseball bat. It’s a very sharp, impulsive
sound, very nice sound if you’re behind the
team who’s batting. So a click, that
baseball hitting the bat, doesn’t happen for very long. A click can be
infinitesimally short. The time that the baseball
is in contact with the bat is pretty short. And if it’s very short
in the time domain, then you have all frequencies. So it’s another example of a
broadband or broad spectrum sound. If the click is
infinitesimally short, the spectrum is completely flat. Those are some common
auditory stimuli. Let’s go through some
more complicated, and maybe more
interesting, sounds. Well, all of us like to
listen to music, right? So here are some examples
of musical sounds. This is a piano keyboard. And here is the spectrum
or frequency representation of what you get when you strike
one key on the piano keyboard. So that’s one note. Well, sure, it sounds
like one thing, but you have a whole bunch
of different frequencies that go along with it. And why is that true? Does anybody know? Why do you get a whole bunch
of different frequencies when you strike a key
on the piano keyboard? Yeah? AUDIENCE: Isn’t it vibrating
all along the length so there’s different
wavelengths? PROFESSOR: What’s vibrating– AUDIENCE: It’s not– PROFESSOR: In the piano? AUDIENCE: It’s not– it’s like
an infinitely small portion of the string. It’s the longer string. It’s parts that are
shorter still vibrating. PROFESSOR: Yeah,
you’re getting there. In the piano, the string
is fixed at one end, and it’s a long string. It [? fits ?]
[? in ?] the other. And your key that you press
down makes a hammer go up, and there’s a bunch of linkages. And eventually, the hammer
hits that string somewhere. And the string, it’s fixed here. It’s not going to move. It’s fixed here. It’s not going to move. But in between those
points, it can move. So it can vibrate like this,
or it can go up and down. It can also vibrate like this. You can have what’s called
a node in the middle. In fact, if you put your finger
right here and fix that middle, it wouldn’t allow
the string to vibrate in this uniform fashion. But it would allow this half
to vibrate and that half to vibrate. This node is sort of a
constraint for this string. It can also vibrate like this. I wish I had a different color. Over here? Great. OK. You can also have the
string vibrate like this. OK. And it can vibrate in many,
many different patterns. I’ve just drawn a few. What’s interesting is
that this length is twice as long as this length, which
is twice as long as this length. And what would you expect the
time of those vibrations to be? Well, the big long thing is
going to vibrate pretty slowly. That’s what’s called the
fundamental frequency. The thing that’s vibrating
in two parts, it’s shorter and it can vibrate faster. In fact, it vibrates
twice as fast. So the first harmonic
is twice the frequency of the fundamental,
and so on and so forth. You can get from the
physical characteristics of the vibration of that
string a whole bunch of different vibration patterns. And they’re usually a harmonic
series– twice, three times, four times, five times, six
times– the fundamental, just because of the physical
characteristics of vibration of the string, and
the wind column in the case of an
Alto saxophone. When you hear that one
note hit by the hammer, all of these vibrations
are happening at once. And so that one sound
sounds like one thing. Musicians will say it sounds
like a note– A above C. But you have a whole bunch
of different harmonics in it. What is pitch? Pitch is very
interesting to people who study the auditory
system, to musicians. Pitch is that attribute of the
sensation, auditory sensation, in terms of which sounds can
be ordered on a musical scale. Let’s say I didn’t let
you see the keyboard, but I recorded the sounds, and
I press some sounds down there, some in the middle, some way up
here, some way at the high end, and I gave you 20
different recordings, and I said, well, make
a ranking of them. Put these down low. Those are number one and two. Put these in the
middle– those are number 10– up to the high end. The highest one is 20. You could do that. The ones that were
down low would be called those with low pitch. The pitch of a pure
tone, of course, depends on the frequency. That’s as if you
were just giving one. If you move that around,
up high end frequency, it sounds like a really
shrilly, high-pitched sound. If you move it down low, it
sounds like a real low sound. The pitch of a complicated
sound– that is, with many overtones
and harmonics– depends strongly on the
fundamental frequency. But sometimes, the fundamental–
for example, in this guitar sound– is pretty weak. And in some cases, you can
take it out altogether. The pitch doesn’t change
that much, surprisingly. So somehow, the ear knows
by this pattern of spectrum that there should be a
fundamental [INAUDIBLE] that can stick it back in. So that’s what pitch is. Another sensation that
musicians often talk about is the timbre of a sound. And the timbre is the quality or
the identification of a sound. It relates to the
highest harmonics here and the pattern
of this harmonics. For the piano, it’s starting
big and sloping down. For a guitar, it’s
starting small, sloping up, and then sloping down. The timbre is what allows you
to identify that sound that you heard as a piano. We can all hear a piano
and say, that’s a piano. We can all hear a guitar
and say, that’s a guitar, or that’s an electric
guitar, because its pattern of harmonics, its fundamental
harmonics, differs. That’s how we identify
sounds is by their timbre or their spectrum, if you will. Those are pretty
complicated sounds. What do I have next? I have a demonstration. This one is called
Canceled Harmonics. And it’s a very
nice demonstration to illustrate the
idea that I said, when you have all these
harmonics go on together, it sounds like one thing,
one note, one sound. But if you take some
of the harmonics out and put them back
in, you’re aware of that taking out and putting back in. So what they’re going
to do is a complex tone is presented, followed
by several cancellations and restorations of a
particular harmonic. And let me show you
what complex tones they’re going to give you. It’s simply this square wave. This is what you’re
going to be listening to. It sounds like
[MAKES BUZZING NOISE]. It’s not very musical at all. And it has a fundamental
and a whole bunch of harmonics, an
infinite number. When that complex goes on at
once, you’re going to say, that sounds like a nasty sound. It sounds like a buzz almost. Then they’re going to take this
one harmonic and pull it out, and then they’re going
to put it back in. As they do that,
you’re going to say, well, that sounded differently. When it was out and
when it was back in, I could hear that
thing going in and out. And then they’re going to do
that for the second, third, and fourth on up to, I
think about 10 or so. Even though this
whole constellation sounds like one sound, when they
pulse these things in and out, you can tell. Let’s listen to
the demonstration, and let’s see how many times
they’re going to do it. This is done for
harmonics one through ten. Canceled Harmonics. A complex tone is
presented, followed by several cancellations
and restorations of a particular harmonic. This is done for
harmonics one through 10. OK. Could everybody hear when
this complex went on all at once it sounded like one sound? Then when individual components
were taken out and pulsed back in, you could identify them. Your ear is very good
at distinguishing the various frequencies
in a complex spectrum. All that message is sent to the
brain as individual channels, and the brain somehow
perceives that when everything is going on at the same
time, that’s one sound. It’s really not of
interest to the brain that the string is
vibrating a whole bunch of different frequencies. It’s that there’s
one string vibrating. But if you took out one of
these modes– in other words, if I put my finger here and
the fundamental goes away, you ear is very good
at detecting that. And it sends a
message to the brain that the fundamental
is no longer there. And the brain says, something
different has happened. So the ear is very
good at recognizing those different characteristics. The brain is good at putting
them back together and saying, they started at one
time, so it’s one object. Questions about that so far? Now, the last type of complex
sound that I want to cover is speech sounds. And I want to save most of this
for the end of the semester when we talk about the parts
of the auditory system that are active in distinguishing
different speech sounds. But let me just– because
we’re talking about sounds and complex sounds, talk
about speech sounds. This is a diagram of
your vocal cavity. Way down at the bottom
here, you get air from your lungs that goes
through your trachea. And in the trachea, there’s
these vocal cords, if you will, that are scientifically
called the glottis. The opening in between
is the glottis. So air can come out, or if
you use muscles associated with your vocal cords,
you can close that off. As the air comes out from here,
it moves those vocal cords back and forth. And they hit each
other, and they open up, and they hit each
other and open up. And as they do that, they
interrupt the airflow and they allow it
to pass through. And they interrupt it, and they
allowed it to pass through. And if you were to
put a microphone way down your trachea right
above those vocal cords, you would see this
time waveform. The pressure would go up right
as the air pressure is coming from the lungs when the
vocal cords were open. When the vocal cords are shut,
there’s no pressure there, or it’s just
atmospheric pressure. So this opening and closing
of the air through the glottis forms this very
complicated waveform. If you look at the
spectrum of it, it has a whole bunch of
different frequencies. The lowest of the
frequencies is the frequency that these things are
opening and closing. But there’s a whole
bunch of harmonics. It’s a very
complicated spectrum. The upper part of
your vocal tract is what’s called the filter. And it serves to emphasize
some of those harmonics and de-emphasize others. And the filter function
is indicated here having three peaks. Those peaks are
called formant peaks. They have to do with the
shape and dimensions, lengths and widths of your
upper vocal tract. What’s kind of neat is by
manipulating, let’s say, where your palate is,
and where your lips are, and where your tongue is, you
can change that filter function by using the muscles
that move things around in your upper vocal tract. And after you’ve filtered
this complex spectrum, you come out with
a function where some of these spectral
peaks are emphasized and some are not emphasized. And here’s the
function that you would get right outside in the
air outside the front of your mouth. This is the time wave form here. Here are some examples
of manipulation of your upper vocal tract. For instance, here the
lower part of the mouth is moved way up high, and it
produces an acoustic spectrum where you have a big f1. And f2 and f3 are
small, and they are way up high in frequency. Contrast this with when
the bottom of your mouth is lowered and moved backward. Here, F1 is even lower. F2 is quite low. And F3 is moderately low. And these are, of
course, the way you pronounce different vowels. We can all say these two vowels. This is the vowel
“i” as in “hit.” Everybody say that– hit, hit. You can kind of feel that
the lower part of your mouth is moved upward. Whereas if you do something
like this– “a” in call. Call– everybody say that. Call. You can feel the lower
part of your mouth dropping down as indicated
here in making a big cavity, whereas here the
cavity is very small. It changes the
acoustic spectrum. Our ears pick it up. And our ears are very
good frequency analyzers. And they say the spectrum here
sounds like hit, because you’ve learned to associate that
spectrum with that vowel. This is a different spectrum. Our ears pick it
up and they say, that’s the vowel
“a” as in “call.” That’s how speech
sounds are formed. At least this works very
well for vowel sounds. It doesn’t explain things
like consonant sounds, which of course are many
different kinds. There’s stop consonants
where your lips close down before you utter
the consonant “p.” So “p,” everybody
close their lips down, and then all of the
sudden you open it up. It’s a completely
different thing. That’s not modulating
the spectrum. That’s modulating
the time pattern. These vowels are
distinguished by their different
spectral patterns, which is picked
up by your years. So I just thought you’d
want to know about that. Speech sounds are among the most
complicated acoustical sounds because of the number
of frequencies involved, the formation, and of
course the perception of telling, for example,
one vowel from another. Let’s shift gears and move on. And instead of talking about
the physical characteristics of sound, let’s talk
about how we hear sounds. We’re only going to get as
far as the auditory periphery today, but let’s just define it. The auditory periphery is this
whole structure indicated here, and it’s usually separated
into three parts– the external ear, the middle
ear, and then the inner ear. Those are the three
very big divisions of the auditory periphery. In the external ear,
you have your pinna. Here’s your pinna. You have the ear
canal, which goes down about three centimeters
inside your head, and it ends up at this
yellow structure here called the ear drum. Tympanic membrane is
the scientific term for the ear drum. That’s the end of
the external ear. The middle ear is an
air-filled cavity. So we’re still talking
about sound in the ear. In that middle ear cavity
are three small bones. They’re called ossicles. I think– yeah, here we go. And in high school
biology, you probably learned them as hammer,
anvil, and stirrup. But the scientific names are
malleus, incus, and stapes. And they convey these sound
vibrations of the ear drum. When sound hits the ear
drum, it causes it to move. And these bones are linked
right onto the eardrum, and they’re linked
one to another. The ear drum then
moves the bones, and the bones finally end up,
in the case of the stapes, in the inner ear. So that’s where the
inner ear begins. I have a demonstration
of ossicles, and I’ll pass them around. These are ossicles
from a guinea pig, and they’re glued to the
bottom of this little vial. And I made a crummy
drawing of them. But if you hold this vial so
that the piece of tape on it is downward, you
get this view here. You have the stapes. And I didn’t list
the other ones. But in the guinea pig,
the incus and malleus are fused, so they
can be considered one. This is definitely
part of the malleus. But I don’t know where the incus
ends and the malleus begins. If you had an ear drum, it
would be this dashed line here. So let me just
pass these around. And you can probably
appreciate from my diagram how the high school biology name
for the stapes got its name. It’s the stirrup. What’s the stirrup? Does anybody know
what a stirrup is? Yeah. When you ride
horses, what is the– AUDIENCE: You put
your foot in it. PROFESSOR: You put
your foot in it. And that’s why cowboy
boots have a nice big heel, so your foot doesn’t go
all the way through it. It sticks in your heel. So this is the stirrup. You put your cowboy
boot right in there until your heel hits
this foot plate. That’s pretty obvious
how that got its name. It’s the foot plate
where you put your foot. Your foot goes right on that. And that foot plate
is the beginning of the next division,
which is the inner ear. And by the way, I
should point out before I forget– what is the
smallest bone in the body? All answers are given. The stapes is the
smallest bone in the body. Why? It’s got to move. And the lousy little sound–
it’s this tiny little ear drum. Remember, the ear
drum is basically a tiny, little
thin piece of skin. It’s like Saran wrap. When your doctor looks
down your ear canal, that doctor can look right
through the ear drum. It’s so thin. It’s like plastic wrap. The doctor can look into
the middle ear and say, so much fluid in there. You’ve got a middle
ear infection. Or they can say,
middle ear looks good. You’ve got some other problem. That’s what we’re looking at. They’re looking with their
otoscope and a light right through the ear drum
into the middle ear. And that whole middle
ear drum and the ossicles have to vibrate when there’s
a tiny little sound like a pin drop. The pin drops right
there, and you can hear it because these things are so
light and flexible that they can vibrate– and so small. The stapes foot plate
ends up at the cochlear. And the cochlear is the
main part of the inner ear. And cochlear, as it
says here, gets its name from the Greek word
kochlias, which means snail. And certainly, the inner ear
looks like a snail shell. And in the inner
ear, here’s where sound changes from sound in air,
or maybe sound in the bones. The inner ear is
filled with fluid. And inside the inner ear are
these wonderful receptor cells for hearing and the beginning
of the auditory nerve. Here’s the auditory
nerve that’s sending messages centrally
into the brain. So the brain would be
beginning right here. This whole structure
here, all this gray stuff, and even the shell of
the cochlear is bone. And it’s your temporal bone. The temporal bone is the
hardest bone in the body. You can have a severe
blow to the head and that temporal bone will keep
all these structures intact. It’s very, very hard bone. Surgeons at our
hospital do a lot of drilling with
the dental drill. They get down to these
important structures, because they have
to manipulate them. These loops here are
part of the inner ear, but they are part that is
sensitive to vestibular sensation. So those loops are called
the semicircular canals. They are almost circular. They are in the three
planes, X, Y, and Z. And when you rotate your
head, let’s say, side to side, one of those can move. And the receptor cells in
it can sense that movement and detect that
your head had moved. And it’s very important,
because if you want to keep your eyes
fixated on one point but move your head,
you can do that by the vestibulo-ocular reflex. The neurons from this
the vestibular system send messages into
the brain stem, and eventually they go
through coordinating centers into the motor neurons for
the extraocular muscles, which can, of course, move
your eyes when you want to do a [? secade ?]
or pursuit, or they can keep your eyes stabilized,
which is moving them with respect your head even
though your head is moving. But we’re not going
to talk about those. Let’s talk about the
function of the middle ear and the external ear. That’s what we’re going to talk
about for the rest of today. I have a model. Let me just pass
around this model. I think we passed around before
on the first day of class, but you can look at it
again, because we’re going into more detail
today on this structure. This comes apart. Here’s your pinna. Here’s the long ear canal. Here’s the ear drum. And if I tilt this here,
you can see the structures we’re talking about in the
inner ear– the cochlear, the semicircular canals, and
this yellow structure here is the auditory nerve. It’s been going into the brain. The brain is cut off here. This is the
eustachian tube, which is a way to vent the
air-filled middle ear. So you want to
purge that with air. If you go up hiking
in a tall mountain, the barometric pressure
outside gets lower. You want to equalize
that in your middle ear. You open that eustachian
tube, usually by swallowing. The ossicles are here. And if you take
out this inner ear, the stapes is fixed with it. So you can see the stapes. In terms of size, this whole
inner ear– the cochlear is about the size of an
aspirin tablet in a human. It’s about that size. OK. Let’s pass that around. OK. What is the function
of the middle ear? Why do we have
these three bones? Why do we have the eardrum? Why doesn’t sound come right
in and strike the inner ear itself? Well, it turns out
that if you look at the physical characteristics
of sound in air, and you want to get
that airborne sound to sound in water,
different medium. So this is fluid or water. This is air. Sound is coming in
here, and you want to get it into the fluid of the
inner ear, which is essentially water. If you don’t do anything and you
have the sound coming in here, most of it bounces back off. In fact, 99.5% of
the energy of sound in air at a fluid boundary is
reflected back into the air. So if you’re in a
boat here– I didn’t draw this right– you’re in
a boat here, you’re fishing, you’re talking to your buddy
in the back of the boat and you say, pass me another
beer, and your buddy says, be quiet, you’ll scare
the fish– actually, the fish can’t hear you. Because most of the energy in
your saying “pass me a beer” bounced right back
off into the air. So how does the auditory
system deal with this? We want to listen very
carefully to a pin drop, but most of the energy
bounces back off at this boundary
between air and fluid. That’s the job of
the middle ear. Here is how the
middle ear moves. This is a nice movie
made by Heidi Nakajima at Mass Eye and Ear Infirmary. This orientation is a
little bit different, but this is the ear drum. This is the malleus, the
incus, and the stapes. Together, they’re
the middle ear. I said this inner ear
is the cochlear here, and it’s encased in bone–
fluid encased in bone. So how does this stapes work? Well, there’s a little
window in the bone. It’s called the oval window. And the foot plate of the stapes
pushes on that oval window. It’s not indicated
here, but it’s right underneath this oval part. There’s another window
called the round window. That’s indicated by blue there. And it’s just a
pressure relief point, because if you pushed on fluid,
it would push back to you. Fluid is relatively
incompressible. So this pushing in means this
membrane over the round window can push out easily. So it’s easy to push
in and pull back, because this membrane can give. As you can see, the
motion of these bones is coming into the
fluid quite nicely and changing some membranes
inside the inner ear. The job of the middle ear is so
that most of that sound energy gets into the fluids of
the water of the inner ear. How does it do that? The primary way is
by changing area. The eardrum is this big drum,
and the stapes foot plate is this much lower in
area or smaller structure. And there’s some formulas here. p1, a1, those pressure and
area at the tympanic membrane, equals p2 a2 where the same
characteristics at the stapes foot plate. So when you decrease the
area a lot, a2 goes way down. p2 has to go way up. So that’s then the main
way that the middle ear allows sound and air to
go into sound and fluid. The engineers would call
this impedance matching. And they would say that
when you change media, the impedance of one medium
being different from another means that most of the energy
is going to bounce back off. If you have a device
here like the middle ear to make the impedances more
matching, much of this energy is going to then go through
the boundary from one medium to the other if
you match the impedances. And one way of
matching the impedances is to change the areas. Another way– and this may
be the reason we have three and not just one middle ear
bone– is by a lever action. So this is kind of like a
lever where the fulcrum is off to one side, not
right in the middle. And you can obviously
get force amplification from a lever action. A third mechanism
might be a buckling of the tympanic membrane. And you’ll have to read– I’m
not an expert on that at all. I’m not even sure if that’s
even in vogue these days. But these actions are
much less than the change in area offered by the eardrum. So what happens when a patient
comes into the Massachusetts Eye and Ear Infirmary,
and for some reason, either via an accident or
a developmental problem, they don’t have an
eardrum, and they don’t have these three ossicles. So the sound goes right
in from the outside and strikes, let’s say, the
round window of the cochlear. Are they deaf? Well, no. They have a hearing loss. Some of the energy gets
through into the fluid. How big is their hearing loss? Well, this is the
so-called audiogram that’s generated when
you visit a hospital and you complain that your
hearing isn’t so good. They send you down to
the Audiology Department. They put you in a testing booth. They put earphones on, and
the tester goes outside so they don’t make
any extraneous noise. And they say, raise your hand
when you can hear a sound. So they test your hearing– this
is the so-called audiogram– and plot it on the
y-axis as the amount of hearing loss in decibels. It’s just the way they plot it. And plot it on the
x-axis as the frequency. And they typically
test 2,550, 1,000– which is abbreviated
here 1k– 2k, and 4k. They typically don’t test the
extremes of the human hearing. They test the middle range. This is the range over which
most speech sounds are made. And that’s the most
important for most people. When they say, I
can’t hear very well, it means they can’t understand
somebody when they’re speaking. And this is the
audiogram from someone who lacked a middle ear. And this 40 dB here– across
all the different frequencies, approximately 40 db– is
the amount of hearing loss they have. So if you go back
to the audiogram that we had in the first
slide of today’s lecture, everything would be
lifted up by 40 dB. You have a 40 dB hearing loss. You’re not deaf
at all, but that’s a moderate to severe hearing
loss, a 40 dB hearing loss. You might have
problems– you certainly would have problems
hearing a pin drop. You might have problems
hearing a telephone ring if it were on the
other side of the room. You might have problems
with conversation. A treatment to that would be
several types of treatment. The surgeons in the Ear,
Nose, and Throat Department at Mass Eye and Ear
could reconstruct your middle ear
and your eardrum. They could use a skin
flap, a piece of skin taken from somewhere
else on your body, put it in the place
of the eardrum. They could use some either wire
or Teflon or plastic pieces that could connect that
eardrum into the oval window of the cochlear. So they can reconstruct the
middle ear fairly easily. If the person doesn’t
want to have surgery, they can have a hearing aid. Essentially, you have a
flat frequency loss here. So put a device in
the ear canal that boosts every single frequency
by 40 dB, amplify the sound. So a hearing aid
works pretty well for these people with this
type of a hearing loss. This type of a hearing loss
is called a conductive hearing loss, because it’s in
the conductive mechanism to conduct the sound
from outside your body into the inside of your body. It’s a conductive hearing loss. So that is the job
of the middle ear– to ensure efficient
transmission of sound in air into the
fluids of your body. And without it, you have a
moderate to severe hearing loss. There’s a disease
called otosclerosis. “Oto” meaning hearing. My department at
Harvard Med School is otology and laryngology. Otology and laryngology. And sclerosis means hardening
or rocky or bony growths. And the surgery that
happens– sometimes around the stapes, bony
growths can grow around it and fix the foot plate so
that it can’t vibrate anymore. So what’s done for that is
you take out the stapes, you take off the bony
growths, and if you just put the stapes back in, often
these bony growths grow again. So actually you take
it out and replace it with an artificial stapes. And the operation is
called a stapedectomy. The “stape” and “ectomy”
means taking it out. You replace it
with a prosthesis. It’s a very successful
surgery for otosclerosis, which is a conductive
hearing loss. That’s the job of
the middle ear, and that’s relatively easy to
treat when there’s a problem. Is there a function
of the external ear? Well, a lot of textbooks say the
external ear funnels the sound into your ear canal. But there is another function
of the external ear that’s more on the lines of localizing
sounds using your external ear. These are examples of
external ears– our pinna. Everybody has a
slightly different one. Who is this historical figure? Anybody? He was a president
of the United States. LBJ, President Johnson. He was always caricatured by
the political cartoon guys with these huge
ears, big pinnae. Everybody has different
shaped pinnae. It turns out that the external
ear can help you localize where sound is coming from. Well, how can it do that? Well, if you have
a pinna and you do this interesting experiment–
you take a microphone and put the microphone
inside here. So here’s the pinna. Here’s the ear canal. Put the microphone out
here, and start out with a completely flat
spectrum, broadband noise. The noise is absolutely
flat so that it has equal energy at
all the frequencies. You measure it out there, and
then you move your microphone down here in the ear canal,
maybe near the eardrum , and measure the spectrum again. So this is plotted
in terms of gain with respect to free field. Free field is out here. Free field means basically in
the room or in the environment. Now we’re going to
measure the spectrum down here and plot the gain. So anything above 0 is going
to be higher than in the ear, and everything below 0
is going to be lower. Let’s look at this
solid curve here, which is minus 15
degrees elevation. Elevation of a sound source– if
it’s straight ahead, it’s zero. If it’s minus 15, it’s
15 degrees below zero. If it’s above zero, it
could be 15 degrees. On this case, it’s 7.5 and 30. So elevations that are
positive are above you. As that sound
source moves around from being below you to above
you, its spectrum changes, the spectrum way down
here at the ear drum. And in particular, there
are some very sharp dips or nulls in the spectrum
that move around. It’s thought that you can
use those nulls as a cue to where this sound is. Now, what causes those nulls? Well, because the pinna
is very complicated, you can imagine that some sound
comes in and strikes the pinna and reflects off it. And maybe it reflects–
excuse my artistic abilities here– maybe it reflects
into the ear canal. Contrast that with other
sound that comes straight in. Eventually, these two sounds
are going to meet up at a point. And let’s say this sound
taking a longer time path went through half of its cycle. So now this sound,
when it’s starting to go a negative
pressure, meets up with this sound, which came
straight in and is starting to go in a positive pressure. Positive plus negative
could sum to zero. And the geometry has
to be just right, and the frequency
has to be just right. But it can be just right
at a particular frequency, and that’s what
causes the nulls. It’s just a physical
characteristic of two sound sources meeting up. It is thought,
then, that you can learn the position
of those nulls, especially, to be associated
with positions of sound in space. And that’s what was done in the
researchers’ report for today. These are some data from
four different subjects. They tested the
subject to localize sounds coming from
in front of them. Left and right would be azimuth. That’s plotted on the x-axis. Up and down would be elevation. That’s plotted on the y-axis. And they move to sounds
around to different places, and they said to
the person, tell me where the sound is coming from. The answers that
the subjects gave are in these solid, thick lines. The real positions were
on the thinner lines. And each big individual data
points are the small points, and the average data
points from the subject are the big points here. So these subjects, when given
a checkerboard of locations, they could pretty faithfully
tell the investigators where a sound was coming
from, both in elevation and in azimuth. These are data from
four different subjects. What was done in the experiment
is distort the pinna. How are we going to do it? Well, we could move
our ear a little bit. What they did was they
put in a little clay mold in parts of the
pinna to change the shape, and they did that on both sides. As soon as they did
that, these are now the answers from the subjects. Terrible in terms of
elevation sensitivity, determining where
a sound is coming from in terms of
different elevations. Still pretty good in azimuth. There are other queues
for sound azimuth that involve using
two ears, which we’re going to talk about extensively
later this semester. The elevational
localization was completely disrupted when the pinna
shape was disrupted. Have these subjects go out for
a few weeks, come back, get tested again. They re-learned
with the pinna molds in how to localize sounds. This is an example of
re-learning or plasticity. Now the pinna cues
had different nulls because the pinnas were
shaped differently. They could re-learn
these new cues and associate them with the
same old changes in elevation that we had before. So that’s why it’s called
re-learning sound localization with new ears or new
or distorted ears. So this is an example
then, of subjects learning to associate these new cues
with the old sound localization positions. So that’s the take home message
from this research report OK questions I can also
do I get on Wednesday, we’ll talk about the inner ear.

2 Comments

Add a Comment

Your email address will not be published. Required fields are marked *