Propagation of Sound Video Lecture - Class 8

check out this speaker if we plug it in
it makes sound the way the 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 speakers
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 eardrums start
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
travelling 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
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 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