Now that we've discussed
the basic properties of waves as well as periodic motion.
We're ready to discuss a particular example
of periodic motion and waves which is sound.
So let's start with talking about the physical basis of sound.
How we measure sound,
as well as some of the basic properties of sound.
Again to contextualize what we've done
is covered many of the mechanics
as well as a few applications including fluids and electromagnetism.
Since we've introduced waves,
we're going to talk about sound before getting to light
and then again we'll get into the smaller structure
and the atoms and molecules.
But first with sound,
what we're going to do is first in this lecture talk about the basics of sound.
Again, just how it's measured,
how it's talked about and then we'll talk about
some more complicated sound dynamics
and how to measure a more complicated properties of sound.
But first with the basics, what is sound?
We can ask ourselves we're hearing all these things all the time
and many instruments and music is obviously more and more prominent
and important to our lives but where is it coming from?
How does it get from one place to our ears
and how do we make particular kind of sound and different frequencies
and putting all those altogether?
What we can do is once again imagine for ourselves maybe a guitar string.
So we grab the string and we pluck it.
What happens is as it moves from left to right
so you can see here in this picture,
guitar strings that's pinned at the bottom and pinned at the top.
It's going to vibrate and move left to right.
As it does that it pushes the air as it moves towards the air
and it does what we say compress the air.
We call it a compression.
So this is a compressed amount of air
and then as the string moves back away from the air.
We have what's called a rarefaction making the air less dense
and so a compression makes the air more dense as it pushes the air
and then when the string moves back we have what's called a rarefaction
which means less dense air.
And as the string vibrates very, very quickly
it creates many, many, many of these compressions
and rarefaction is dense and less dense waves of air.
What we can see here,
is just based on our definition of the two different type of waves.
This is a longitudinal wave
because the motion of the molecules is left and right in this picture
and the motion of the wave is also left to right,
and so sound is a longitudinal wave.
The speed of sound as we talked about with any wave
only depends on the medium through which the sound is traveling.
And so the sound is just traveling through air,
it always has a particular velocity
and so you can always know how quickly sound is traveling
without having to know how fast the source was moving
or whether it was made by a car or anything moving fast or slow.
The speed will always be the same.
The speed for sound and air is about 340 meters per second.
It turns out that again since this speed depends on what medium you're in,
what you're traveling through.
Sound actually travels much more quickly when you go to liquids or solids.
And there's difference in the speed sound through different materials.
It turns out there's very important not only to the sciences,
we use this as different vibrational waves
or sound waves travel through the earth.
Whether it's caused by earthquake or something else
and we can measure things about the internal structure of the earth
and in fact this is how we know what the structure of the earth is.
But aside from the physical sciences
we also can use this effect to study the human body.
We can send little sound waves into the human body
and then we can see what comes out
or what comes out the other side and listen to it
and by just simply doing the kinds of math and physics
that we're introducing here.
We can analyze whether there might have been more solid structure
or less dense structure
just based on how the waves of sound move through the body.
And so there's actually a pretty dramatic clinical significance
to the effects that sound has
as it moves through different kinds of mediums,
different kinds of materials.
The energy from a sound wave as the sound moves out
from the source of that sound is going to be spread out.
It's going to be spread out across the surface area of the sphere.
So you can think about the sound emanating from some source
no matter what it is, that's a motion of a molecule,
its vibrating molecules and those molecules can bump around
in anyone of the three dimensions
and so this energy is being spread and propagated in all three dimensions.
When this happens because the energy has to be spread out
across the surface area of the sphere as its traveling outwards.
This energy is also conserved and so what we can say
is that as the wave, the sound wave travels in all three dimensions
and we know the surface area of the sphere
depends on the square of the radius of that sphere.
We can also say that the intensity or the energy of that sound
drops as the square of the distance that you are away from the sound.
So for example if you move twice as far away from a source of sound,
you would in fact sound one fourth as loud
because again the intensity of the sound
will drop with the square of the distance
rather than with the distance directly for the reasons we just described.
The quartering of this sound being as loud
is actually very important for the way our ears work.
The way humans perceive sound is by distance.
So if I move twice as far away,
the way I would perceive the sound is so that my brain and I know
that the source of the sound is twice as far away.
But we just said that the intensity of that sound how loud that sound is,
is one quarter as loud.
So there's difference the fact that we want to perceive sounds
based on distance so that we can know how far away something is.
Comparing that to the fact that the energy is only a quarter as loud
has some ramification for how we discuss
the intensity of the sound coming to us
and for that reason we need a measuring scale
that expresses this as a power law.
So that we reiterate that
because it might be a little bit of a complicated point.
My perception of sound will depend on my distance away from the sound.
So for example if I'm next to somebody who's talking
and then I moved twice as far away as I was before.
I would want to perceive that distance correctly.
If I close my eyes I would want to be able to know where that person is
but this is added discrepancy with the actual energy of the sound
as it's leaving the person as they speak
because we just described how that is going to drop with the power law,
a square of the distance.
And again for that reason
we have sort of a more complicated way of having to describe sound
when we measure it.