Hi. It's Mr. Andersen. And today I want to talk about the science of sound waves. Sound
waves differ from electromagnetic waves in that they require a medium for the wave to
travel through. In other words the sun, the light from the sun gets here very quickly,
but the sound from the sun simply can't travel through the vacuum of space. So to emphasize
that what I want to do is show you what the last scene of Star Wars would look like if
they really followed the rules of science. So let me shrink my head. And let's watch.
"You're all clear kid. Now let's blow this thing and go home." Now the reason why you
wouldn't hear the Death Star exploding is that there's nothing for those sound waves
or compressional waves to travel through. And so that's just one of the misconceptions
that a lot of people have about sound. First of all then it's a mechanical wave. So it
requires a medium to move through. If there's nothing there the sound waves can't pass.
What is it traveling through right now? Well my vocal cords are vibrating the air, which
is vibrating the air molecules, vibrating the microphone. It's picking up that sound.
If we're talking about water, can it move through water? For sure. In fact it moves
more quickly. Sound waves will travel quicker through water because it's more dense. If
I were to hit it down on the railroad track or railroad tie, the sound waves would travel
very very quickly through that because the steel is very, very dense. And so again it's
a mechanical wave. And it's compressions that move through that. Next thing you should know
is that it is a longitudinal wave. In other words waves can move like this. We call those
transverse waves. Or it can move through this, longitudinal waves. And so if I click the
button and we watch the waves traveling, it's actually moving or the disturbance is moving
in the direction of the wave. In other words when I talk right now, if you could see the
sound waves coming out of my voice, it would be a three-dimensional compressional wave
that's moving out. But it's vibrating the air in the direction that it's actually moving.
Next property is something called pitch. So when we hear sound we talk about pitch. What
is pitch? Well pitch is really frequency. Frequency is how fast the waves are vibrating
or oscillating. And so in science we talk about frequency. But in sound or in music
we usually talk about pitch. What happens if I increase the frequency? I increase the
pitch. Amplitude or how big the waves are is going to tell us the loudness of that wave.
And both of these things are maybe a little easier to understand if we kind of play around
with it. So this is a piece of software called ToneGen. And what I can essentially do is
I can play a note. So let me play a note. This is going to be a vibration at 261.6 hertz.
So when I play that what you're hearing is a vibration of the sound waves. In this case
it's coming from the speakers on your computer. It's vibrating the speakers at a rate of 261.6
per second. Now if I turn down the amplitude of those waves you don't hear it anymore.
In other words when I make the waves or the compressions of the speaker smaller, you can't
hear it. And so the amplitude of the wave refers to the loudness. Let me turn it up
again. But the frequency of the wave tells us what the pitch is that we hear. And so
we can increase the pitch. Well let's quickly double that. So double 261.6 would 523.2.
And so now we would have a higher pitch. And so that's a note that is is twice the frequency
of the wavelength we had before. And so instead of being middle C this is a C that is up one
octave. So let me stop that. And let's go back to our middle C. Which is not quite as
annoying. And so we play that. And so this would be a sine wave. So it's a smooth oscillation
of the wave. We also can have different waves that we can set up. So let's try a square
wave. Sounds a little bit different. Or this would be a triangular wave. So the rate at
which we change that can give us different sounds. This would be sawtooth wave. And then
this would be a impulse train. And I don't even know what that is. So what's the most
pleasing? Well let's go back to the sine wave. Sine wave is going to be just a smooth oscillation
of your speaker back and forth. And we can just increase the pitch now. So if we take
that pitch and increase it. So what are we doing? We're simply making the speaker move
in more and more quickly. And then we're hearing that sound. Now why do you sound good when
you sing in the shower? The reason you sound good when you're singing in the shower is
that waves have a specific wavelength. And each frequency is going to cause a different
wavelength. As I increase my frequency I'm going to decrease my wavelengths. So the length
gets smaller and smaller and smaller. And so these would be higher pitch and shorter
wavelength. These would be lower pitch and longer wavelength. But if you can get the
waves to fit perfectly into the container that they're sitting in, they create a harmonic
or resonance. And so when you sing in a shower, this is Ferris Bueller, the sound waves are
fitting perfectly within that shower. And that's why you sound much better when you're
singing in the shower. It's because the waves fit perfectly. It also explains why you could
have the perfect pitch. And if we increase that amplitude we could shatter a wine glass.
We have the waves fitting perfectly inside that wine glass. So how do we actually perceive
or in other words how does our ear work? Well we have to take a look at it. So this part
of your ear, the outer ear is just funneling the sound down the auditory canal until you
eventually have an eardrum which is going to be on the inside. We take the vibrations.
Right now as we vibrate this window right here, your eardrum, it's connected to a series
of bones that connect to an oval window that sits here on this structure called the cochlea.
It's on the inside of your ear. Why do we go from a really big window to a small window
here? We get amplification. And also we're getting leverage through these simple machines
that connect to that oval window. And so we're actually taking small vibrations here and
making very big vibrations on the inside of your cochlea. Now how does you cochlea actually
work? Well let me take this part of the cochlea. And if we were to stretch it out it would
look like this. And so what do we have? Well what we really have are a series of really
really big, I mean in scale, showers. And really really small showers. And so all along
the cochlea, the air, the fluid in there is going to resonate at a specific frequency.
And so when you hear really high pitches, that's going to be in the really narrow part
of your cochlea. And low pitches are going to be on the really low end. And so there
are going to be frequencies that you can't hear. In other words really low frequency
sounds that you can't pick up. And the reason why is that your cochlea isn't big enough.
If we were to look in a whale cochlea, it's going to be really really big. And so they
can hear very, very low frequency sounds that travel really good in far distances through
the water. And so what is the limit? Well most literature says that we can hear pitches
on the low end at 20 hertz. And at a high end of 20 kilohertz of 20000 hertz. And you
know this if your'e a teenager because you're probably familiar with the mosquito ring tones.
And so how does that work? Well if I play a not like this, this would be 9 kilohertz.
Or this is going to be 9000 hertz. You can hear that and I can hear that. But if we increase
that 10 kilohertz or 11 kilohertz or 12 or 13 or 14 or 15. Right there I can't hear it
any more. That sound. But if you're younger than me you probably can. Or can you hear
it at 16 or 17 hertz? Or 18 hertz? Can you hear it at 19 hertz? 20 hertz? I can't hear
that at all. And it probably has a lot to do with the amount of rock and roll music
I listened to when I was younger. In other words my limit is much much smaller that I
can hear. So I can't hear a lot on the low end or the high end. And that's just because
I'm getting older. And these nerves and the hairs that actually pick that up in my ear
are dropping off as well. So that's the frequency or the pitch. And so we can hear a broad range.
But it's based on this structure right here. And you can see that different parts of our
cochlea are actually able to pick up different frequencies. But the other thing is how loud
a sounds is. And sound, that's not so much the frequency but it's how big they are. And
so we measure that in something called a decibel. And so a decibel goes from 0, which would
be the lowest end. In other words I can just perceive that as a sound or a vibration, all
the way up to 130 decibels which is pain actually comes from that. And this is a logarithmic
scale. In other words this would be 1 decibel, we could think of. But 130 decibels would
be 10 trillion times as much. In other words 1 with 13 zeroes after it. And so we can hear
a really broad range. And we can hear a really broad range of frequencies as well. And that's
all thanks to the amplification of our ear. And the wonderful cochlea. So I hope that
that makes a little bit more sense. And thanks for listening.