1
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- Check out this speaker.

2
00:00:03,126 --> 00:00:04,952
If we plug it in, it makes sound.

3
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(speaker hums)

4
00:00:07,048 --> 00:00:09,309
The way this speaker
creates sound is by moving

5
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the front of the speaker,
which is called the diaphragm,

6
00:00:12,279 --> 00:00:14,085
back and forth rapidly.

7
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Scientists often use the
word oscillation to refer to

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the back and forth motion of an object.

9
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This speaker is oscillating too fast

10
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for the human eye to see,

11
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but if I put a piece of
paper on the speaker,

12
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we see that because the
diaphragm is oscillating,

13
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it's bumping into this piece of paper

14
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and causing it to dance.

15
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The oscillation of the diaphragm will also

16
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cause the air in front of the diaphragm

17
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to move back and forth, but
here's the interesting thing.

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The air in front of the diaphragm

19
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doesn't actually travel
away from the speaker.

20
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The air molecules in front of the speaker

21
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just oscillate back and forth.

22
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So, how can you hear
the sound from a speaker

23
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if the air next to the speaker

24
00:00:51,569 --> 00:00:53,689
doesn't actually make it to your ear?

25
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Well, the reason is
that the oscillating air

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in front of the speaker

27
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causes the air in front
of it to also oscillate.

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This causes the air in front of that air

29
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to start oscillating,

30
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which causes the air in front
of it to start to oscillate,

31
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until finally, the air that's
actually next to your ear

32
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and your eardrum starts to
oscillate back and forth.

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This oscillating air that's
next to your ear is moving,

34
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so it has kinetic energy.

35
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So, it can transfer
energy into your eardrum,

36
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which you can perceive as sound.

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So, this speaker was able to

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transport energy through the air,

39
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without actually having to
transport the air itself.

40
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This is an important enough
fact for me to state again.

41
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Energy is traveling across the room here,

42
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but air itself is not
traveling across the room.

43
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Only the disturbance within the air

44
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is traveling across the room.

45
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If air were being
transported across the room,

46
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it'd be better characterized
not as sound but as wind.

47
00:01:49,710 --> 00:01:52,846
So, this is why we call
sound a sound wave,

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because it shares the
defining feature of waves,

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which is being able to transport
energy through a medium

50
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without having to transport
the medium itself.

51
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Medium is a fancy word for
the material or substance

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through which a wave is traveling.

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Air is typically the medium for situations

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involving sound waves,

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but sound waves can travel through

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all kinds of different materials,

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like water, metal, or
even human flesh and bone,

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and the fact that sound

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can travel through human flesh and bone

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explains something you might
have always wondered about,

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which is, why do our voices sound

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(changes voice) so different
on audio and video recordings?

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The reason for this is that
when we're speaking to someone

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we actually hear two
contributions from our voice.

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We hear the sound wave
traveling out of our mouth,

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through the air, and into our ear,

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but we also hear the
vibration of the sound wave

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traveling through our flesh and bone,

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through our skull, and into our eardrum.

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But on an audio or video recording,

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the only part that's recorded is the sound

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that travels through the air.

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So, when you hear your voice played back

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on an audio recording, you only
hear what other people hear

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when they listen to you.

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So, the bad news is that,

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yes, what you hear on audio recordings

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is actually what you sound
like to other people,

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but the good news is
that most of your friends

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probably don't think it sounds weird,

81
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since that's the only voice
they've ever heard you use,

82
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unless you do (changes voice)
actually have a weird voice,

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00:03:20,367 --> 00:03:22,836
in which case, I'll risk
sounding pretentious

84
00:03:22,836 --> 00:03:25,739
by reminding you that you
shouldn't waste a lot of time

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worrying about what other
people think of you anyways.

86
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(techno music)