Mr.p : What is sound?

Bobby: What is sound?

[intro]

Billy: I think it has something to do with the air around us?

Bo: I think it's just a rhetorical question.

Bobby: No, it's not rhetorical.

Mr.p: Definitely not rhetorical. Let's look at this tuning fork

because a tuning fork is the simplest way to understand sound.

After you bang a tuning fork against something,

the tines of the fork oscillate back and forth in simple harmonic motion at a defined frequency.

It is difficult to hear without something to resonate and amplify the sound.

So, listen as I hold the turning fork against the white board...

Billy: I can hear the tuning fork now.

Mr.p: Often, the sound waves which spread out from a sound source like a tuning fork

are represented by a ring of circles increasing in size.

But what does this animation really represent?

Bobby: Circles are created by tuning forks?

Bo: And aren't there two tines on the fork creating circles so shouldn't there be two sets of circles?

Okay there are two tines of the tuning fork,

and technically there should be two animations shown.

However, we are going to simplify everything by treating the tuning fork as a single sound source.

And no tuning forks do not create circles.

In order to this animation, we need to take a closer look at what is happening to the tuning fork.

The frequency of this tuning fork is 440 Hz.

That means the tines of this tuning fork are going back and forth 440 times every second.

Even when I show a video oF this tuning fork at 32 times slower than real speed,

we still cannot really see the motion of the tuning fork.

Mr. p: On a side note,

BBB:sine note. huh. That's funny. No. Right.

the reason I chose 440 Hz is because 440 Hz is typically what is used as "concert pitch"

and is the note you hear orchestras play when they are all trying to tune to one another.

[piano, violin,guitar, trumpet, singer tuning]

Bobby: What is pitch?

Mr.p: Oh right, sorry, frequency of a sound is interpreted by our brains as "pitch".

A higher frequency means a higher pitch.

and 440 Hz or "concert pitch" is the A above the middle C on the piano.

In order to see what is happening with the tuning fork,

let's actually switch to a speaker creating the same 440 Hz frequency

and then change that frequency down to one where we can see the movement of the speaker in slow motion.

Let's choose 55 Hz which is the A, 3 octaves below concert pitch.

And we are showing the speaker moving back and forth at 55 Hz

slowed down to 32 times slower than real speed

What you see here is the cone of the speaker going back and forth

in simple harmonic motion 55 times per second just like the tines

of the tuning fork were going back and forth at a specific frequency

what you hear as sound is air being compressed and rarified, 55 times every second.

Unfortunately, you cannot see what is happening with the air to cause you to hear the 55 Hz frequency.

So, I am going to show an animation of the cone of the speaker,

represented by the vertical line, moving back and forth in simple harmonic motion and moving the air particles.

You can see the speaker compresses and rarefies the air particles in a simple harmonic motion pattern.

Where the air is compressed, the air pressure is higher than atmospheric pressure,

and where the air is rarified, the air pressure is lower than atmospheric air pressure.

That is why a sound wave is often also called a pressure wave...

Bo, is a sound wave a transverse or longitudinal wave?

Bo: Well...it looks like the direction of the wave propagation

is parallel to the direction of the disturbance of the medium,

so a sound wave must be a longitudinal wave.

Mr.p: Correct Bo. Sound is a longitudinal pressure wave.

Billy: Uh, Mr.p?

Mr. p: Yes Billy.

Billy: I thought air molecules were always moving around in all sorts of directions and

running into one another and running in to objects creating pressure.

Those air molecules are all look like they are moving either to the left or to the right

and it does not really make sense that they would all be evenly spaced out like that.

Mr.p: Oh, good point Billy.

This is a highly idealized model of sinusoidal sound waves moving through the air.

You are correct that the air molecules would move in all directions and be more randomly located.

However, the average position and average velocity of the particles would look like this.

It is easier to understand what is going on when we look at the

Idealized average position and velocity of the air particles.

Alright, we still have not connected this back to the original sound wave animation with all the circles.

So, looking at the speaker and air animation

notice how there are vertical lines of high density or high-pressure air.

Those correspond to crests on a sinusoidal wave which represents the pressure of the air.

Those vertical lines of high pressure, or crests on the sinusoidal wave,

move linearly away from the speaker.

Those crests of high pressure are also called wave fronts.

A wave front is a region of the medium which is all at the same location on the sinusoidal wave.

But those wave fronts are only linear from a very local perspective.

A more accurate representation is that the sound waves move out in all directions from the sound source.

In other words, the sound waves, or wave fronts, move out in a spherical shape away from the sound source.

However, the medium you are looking at to understand sound waves,

this screen, is not three-dimensional, it is a two-dimensional space.

And class, what is a two-dimensional slice of a sphere.

Billy: A two-dimensional slice of a sphere is a...

BBB: Circle.

That's why the original animation is of circles!

Bobby: Yeah.

Bo: Right.

Mr.p: So this is just like the original animation of the circles increasing in radius

and emanating from the sound source.

Those circles represent the crests of the pressure wave

and the crests represent the locations of high pressure or high density air.

Again, these circles, or rather spheres, are also called a wave fronts.

So these spheres are wave fronts of high pressure air which move away from the sound source.

Hopefully this helps you to actually understand this animation which is often used to illustrate sound.

Bo: But, what is sound? I mean, have we even answered the question.

Bobby: Uh, sound is periodic pressure waves traveling through a medium.

Billy, And, for us humans, the medium is typically air,

however, sound can travel through any solid, liquid or gas.

Bobby: And the "pitch" of the 440 Hz tuning fork is your brain's interpretation of that air pressure

oscillating from high pressure, to low pressure, and back to high pressure 440 times every second.

Bo: So, sound is just periodic waves traveling through a medium?

Why did he not just say that at the beginning?

Bobby: Uh...

Billy: I guess Mr.p had more for us to learn than just that.

Mr. p: Billy you know that is always true.

For example, please remember that the net displacement of the medium is zero.

Only the disturbance of the medium moves through the medium, not the medium itself.

You can see that in animation where I have highlighted one set of air particles

which all have the same air pressure.

You can see the air particles move back and forth,

increase and decrease the local air pressure in simple harmonic motion, however;

the air molecules have zero net displacement.

Billy: But if the air is not displaced, what is actually transferred through the air via the sound wave?

I mean those circles which represent crests must mean something is transferred through the air, right?

Bobby: ... Energy. Remember, the disturbance of the medium is the energy stored in the medium.

So, energy travels through the medium, but the air does not change locations.

Bo: Okay, so those crests, or wave fronts,

and the troughs which are between every crest

represent periodic waves of high and low density air

which air transferring sound energy away from the sound source. That makes sense.

Mr.p: There is a term called "pressure amplitude" which is simply the amplitude of the pressure wave.

That means pressure amplitude is the maximum difference between

the local pressure of the air and the atmospheric pressure.

Bobby: Pressure amplitude? It seems pretty clear what that is.

It is just the amplitude of the wave measured using pressure.

In other words, the maximum difference between the highest pressure of the wave

and the average pressure of the wave, right?

Billy: Does that mean a higher pressure amplitude means a larger amount of energy

transferred by the sound wave?...

Oh, and I bet it is louder too.

Mr.p: Correct. Higher pressure amplitude means more sound energy and the sound is perceived as louder,

by humans if it is in the Human Audible Range?

Bo: Wait, wait a second. What is the Human Audible Range?

Mr.p: Oh right, I'm sorry, I haven't talked about Human Audible Range.

So, the Human Audible Range is considered to be from 20 Hz to 20,000 Hz or 20 kHz.

This is a sound at 20 Hz and I will slowly raise the frequency of the sound

so you can hear the full range from 20 Hz to 20,000 Hz.

This is why dog whistles have a frequency above 20 kHz

because dogs can hear those frequencies and humans cannot.

As humans get older, we suffer from presbycusis which is the natural aging of the human auditory system

which begins with the loss of hearing in higher frequencies.

Bo: Wait, if the pressure amplitude is increased at a frequency you cannot hear, is the sound louder?

Billy: Right, "If a tree falls in a forest and no one is around to hear it, does it make a sound?"

Bobby: This is not a philosophy class.

Mr. p: Right. So, I will mention that most sounds are not a simple sinusoidal wave form

like the one created by the tuning fork.

For example, the sound wave created by my voice right now is very different than a sinusoidal wave, however,

it is still comprised of changing air pressures which also has a wave form,

it is just that wave form is much more complicated than a sinusoidal wave.

Billy: Can I see a wave form for my voice? I think that would be cool?

Bobby: Yeah, I want to see mine, I think?

Bo: I want see a wave form for my voice.

Mr. p: .....Okay, one last thing. In the absence of a medium, can sound propagate?

BBB: No.

Mr. p: That is correct. In other words, if you we were to remove all the air from this room,

would y'all be able to hear me?

BBB: No.

Correct, again. Thank you very much for learning with me today, I enjoyed learning with you.

Billy: I feel like, if there were no air in this room,

not being able to hear Mr.p would be the least of our worries.

Bo: Yeah, I bet it wouldn't be in the vacuum that you can breathe.

Billy: Oh Yeah, I forgot about that. The vacuum that you can breathe!