- This is what a sound wave sounds like,
(speaker hums)
but what does a sound wave look like?
Well, the air through which the sound wave is traveling
looks something like this,
but if you want another visual representation of the sound,
we can hook this speaker up to an oscilloscope,
and it gives us this graph.
(speaker hums)
We say that this shape represents the sound wave,
because if we focus on a single molecule of air,
we see that it moves back and forth,
just like a sine or cosine graph.
The horizontal axis here represents time,
and the vertical axis can be thought of
as representing the displacement of that air molecule
as it oscillates back and forth.
The center line here represents the equilibrium position
or undisturbed position of that air molecule.
It we turn up the volume,
we see that the oscillations become larger,
and the sound becomes louder.
The maximum displacement of the air molecule
from its undisturbed position is called the amplitude.
Be careful.
The amplitude is not the length of the entire displacement.
It's only the maximum displacement measured
from the equilibrium position.
Another key idea is the period of a sound wave.
The period is defined to be the time it takes
for an air molecule to fully move back and forth one time.
We call this back and forth motion a cycle.
We measure the period in seconds.
So, the period is the number of seconds
it takes for one cycle.
We use the letter capital T to represent the period.
If we decrease the period,
the time it takes for the air molecules
to oscillate back and forth decreases,
and the note or the pitch of the sound changes.
The less time it takes the air molecules
to oscillate back and forth,
the higher note that we perceive.
An idea intimately related to the period
is called the frequency.
Frequency is defined to be one over the period.
So, since the period is
the number of seconds per oscillation,
the frequency is the number of oscillations per second.
Frequency has units of one over seconds,
and we call one over a second a hertz.
Typical sounds have frequencies
in the 100s or even 1000s of hertz.
For instance, this note, which is an A note,
is causing air to oscillate back and forth
440 times per second.
So, the frequency of this A note is 440 hertz.
Higher notes have higher frequencies,
and lower notes have lower frequencies.
Humans can hear frequencies as low as about 20 hertz
and as high as about 20,000 hertz,
but if a speaker were to oscillate air back and forth
more than about 20,000 times per second,
it would create sound waves,
but we wouldn't be able to hear them.
(sound starts, then stops)
For instance, this speaker is still playing a note,
but we can't hear it right now.
Dogs could hear this note, though.
Dogs can hear frequencies up to at least 40,000 hertz.
Another key idea in sound waves
is the wavelength of the sound wave.
The idea of a wavelength is that when this sound
is traveling through a region of air,
the air molecules will be compressed
close together in some regions
and spread far apart from each other in other regions.
If you find the distance between two compressed regions,
that would be the wavelength of that sound wave.
Since the wavelength is a distance, we measure it in meters.
Be careful.
People get wavelength and period mixed up all the time.
The period of a sound wave is the time it takes
for an air molecule to oscillate back and forth one time.
The wavelength of a sound wave is the distance
between two compressed regions of air.
People get these mixed up
because there's an alternate way to create a graph
of this sound wave.
Consider this.
Before the wave moves through the air,
each air molecule has some undisturbed position
from the speaker that we can measure in meters.
This number represents the equilibrium
undisturbed position of that air molecule.
Then as the sound wave passes by,
the air molecules get displaced slightly from that position.
So, an alternate graph that we could make
would be the displacement of the air molecule
versus the undisturbed position
or equilibrium position of that air molecule.
This graph would let us know
for a particular moment in time
how displaced is that air molecule
at that particular position in space.
This graph shows us that in some regions
the air is displaced a lot from its equilibrium position,
and in other regions, the air is not displaced much at all
from its equilibrium position.
For this kind of graph, the distance between peaks
represents the wavelength of the sound wave,
not the period, because it would be measuring
the distance between compressed regions in space.
So, be careful.
For a sound wave, a displacement versus time graph
represents what that particular air molecule is doing
as a function of time, and on this type of graph,
the interval between peaks represents
the period of the wave,
but a displacement versus position graph
represents a snapshot of the displacement
of all the air molecules along that wave
at a particular instant of time,
and on this type of graph,
the interval between peaks represents the wavelength.