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- This is what a sound wave sounds like,

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(speaker hums)

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but what does a sound wave look like?

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Well, the air through which
the sound wave is traveling

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looks something like this,

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but if you want another visual
representation of the sound,

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we can hook this speaker
up to an oscilloscope,

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and it gives us this graph.

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(speaker hums)

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We say that this shape
represents the sound wave,

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because if we focus on a
single molecule of air,

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we see that it moves back and forth,

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just like a sine or cosine graph.

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The horizontal axis here represents time,

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and the vertical axis can be thought of

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as representing the displacement
of that air molecule

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as it oscillates back and forth.

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The center line here represents
the equilibrium position

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or undisturbed position
of that air molecule.

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It we turn up the volume,

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we see that the
oscillations become larger,

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and the sound becomes louder.

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The maximum displacement
of the air molecule

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from its undisturbed position
is called the amplitude.

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Be careful.

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The amplitude is not the length
of the entire displacement.

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It's only the maximum
displacement measured

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from the equilibrium position.

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Another key idea is the
period of a sound wave.

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The period is defined
to be the time it takes

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for an air molecule to fully
move back and forth one time.

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We call this back and
forth motion a cycle.

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We measure the period in seconds.

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So, the period is the number of seconds

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it takes for one cycle.

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We use the letter capital
T to represent the period.

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If we decrease the period,

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the time it takes for the air molecules

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to oscillate back and forth decreases,

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and the note or the pitch
of the sound changes.

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The less time it takes the air molecules

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to oscillate back and forth,

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the higher note that we perceive.

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An idea intimately related to the period

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is called the frequency.

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Frequency is defined to
be one over the period.

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So, since the period is

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the number of seconds per oscillation,

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the frequency is the number
of oscillations per second.

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Frequency has units of one over seconds,

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and we call one over a second a hertz.

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Typical sounds have frequencies

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in the 100s or even 1000s of hertz.

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For instance, this note,
which is an A note,

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is causing air to oscillate back and forth

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440 times per second.

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So, the frequency of
this A note is 440 hertz.

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Higher notes have higher frequencies,

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and lower notes have lower frequencies.

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Humans can hear frequencies
as low as about 20 hertz

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and as high as about 20,000 hertz,

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but if a speaker were to
oscillate air back and forth

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more than about 20,000 times per second,

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it would create sound waves,

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but we wouldn't be able to hear them.

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(sound starts, then stops)

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For instance, this speaker
is still playing a note,

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but we can't hear it right now.

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Dogs could hear this note, though.

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Dogs can hear frequencies
up to at least 40,000 hertz.

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Another key idea in sound waves

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is the wavelength of the sound wave.

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The idea of a wavelength
is that when this sound

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is traveling through a region of air,

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the air molecules will be compressed

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close together in some regions

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and spread far apart from
each other in other regions.

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If you find the distance
between two compressed regions,

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that would be the wavelength
of that sound wave.

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Since the wavelength is a
distance, we measure it in meters.

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Be careful.

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People get wavelength and
period mixed up all the time.

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The period of a sound
wave is the time it takes

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for an air molecule to oscillate
back and forth one time.

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The wavelength of a sound
wave is the distance

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between two compressed regions of air.

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People get these mixed up

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because there's an alternate
way to create a graph

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of this sound wave.

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Consider this.

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Before the wave moves through the air,

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each air molecule has
some undisturbed position

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from the speaker that we
can measure in meters.

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This number represents the equilibrium

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undisturbed position of that air molecule.

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Then as the sound wave passes by,

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the air molecules get displaced
slightly from that position.

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So, an alternate graph that we could make

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would be the displacement
of the air molecule

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versus the undisturbed position

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or equilibrium position
of that air molecule.

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This graph would let us know

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for a particular moment in time

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how displaced is that air molecule

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at that particular position in space.

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This graph shows us that in some regions

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the air is displaced a lot
from its equilibrium position,

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and in other regions, the air
is not displaced much at all

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from its equilibrium position.

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For this kind of graph,
the distance between peaks

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represents the wavelength
of the sound wave,

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not the period, because
it would be measuring

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the distance between
compressed regions in space.

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So, be careful.

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For a sound wave, a
displacement versus time graph

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represents what that particular
air molecule is doing

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as a function of time,
and on this type of graph,

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the interval between peaks represents

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the period of the wave,

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but a displacement versus position graph

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represents a snapshot of the displacement

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of all the air molecules along that wave

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at a particular instant of time,

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and on this type of graph,

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the interval between peaks
represents the wavelength.