- Check out this speaker.
If we plug it in, it makes sound.
(speaker hums)
The way this speaker creates sound is by moving
the front of the speaker, which is called the diaphragm,
back and forth rapidly.
Scientists often use the word oscillation to refer to
the back and forth motion of an object.
This speaker is oscillating too fast
for the human eye to see,
but if I put a piece of paper on the speaker,
we see that because the diaphragm is oscillating,
it's bumping into this piece of paper
and causing it to dance.
The oscillation of the diaphragm will also
cause the air in front of the diaphragm
to move back and forth, but here's the interesting thing.
The air in front of the diaphragm
doesn't actually travel away from the speaker.
The air molecules in front of the speaker
just oscillate back and forth.
So, how can you hear the sound from a speaker
if the air next to the speaker
doesn't actually make it to your ear?
Well, the reason is that the oscillating air
in front of the speaker
causes the air in front of it to also oscillate.
This causes the air in front of that air
to start oscillating,
which causes the air in front of it to start to oscillate,
until finally, the air that's actually next to your ear
and your eardrum starts to oscillate back and forth.
This oscillating air that's next to your ear is moving,
so it has kinetic energy.
So, it can transfer energy into your eardrum,
which you can perceive as sound.
So, this speaker was able to
transport energy through the air,
without actually having to transport the air itself.
This is an important enough fact for me to state again.
Energy is traveling across the room here,
but air itself is not traveling across the room.
Only the disturbance within the air
is traveling across the room.
If air were being transported across the room,
it'd be better characterized not as sound but as wind.
So, this is why we call sound a sound wave,
because it shares the defining feature of waves,
which is being able to transport energy through a medium
without having to transport the medium itself.
Medium is a fancy word for the material or substance
through which a wave is traveling.
Air is typically the medium for situations
involving sound waves,
but sound waves can travel through
all kinds of different materials,
like water, metal, or even human flesh and bone,
and the fact that sound
can travel through human flesh and bone
explains something you might have always wondered about,
which is, why do our voices sound
(changes voice) so different on audio and video recordings?
The reason for this is that when we're speaking to someone
we actually hear two contributions from our voice.
We hear the sound wave traveling out of our mouth,
through the air, and into our ear,
but we also hear the vibration of the sound wave
traveling through our flesh and bone,
through our skull, and into our eardrum.
But on an audio or video recording,
the only part that's recorded is the sound
that travels through the air.
So, when you hear your voice played back
on an audio recording, you only hear what other people hear
when they listen to you.
So, the bad news is that,
yes, what you hear on audio recordings
is actually what you sound like to other people,
but the good news is that most of your friends
probably don't think it sounds weird,
since that's the only voice they've ever heard you use,
unless you do (changes voice) actually have a weird voice,
in which case, I'll risk sounding pretentious
by reminding you that you shouldn't waste a lot of time
worrying about what other people think of you anyways.
(techno music)