Sound waves are mechanical waves. That means they
require a medium to propagate through. Astronauts
in space can't talk to each other without touching
their helmets. (Or they can use radio).
Sound waves are longitudinal waves. That means the
medium they travel through moves parallel to the wave
itself. That's not always obvious in everyday life
because we can't usually see sound waves in motion.
The audible range for humans starts around 20 Hz.
Even if you have a large speaker making 20 Hz sound,
it's still pretty hard to see the speaker moving.
But suppose you slow down the speaker even more by
inputting a much lower frequency, like 1 Hz? If you
amplify the signal to really drive the speaker to
its full deflection, you can easily see the speaker
move up and down --- one up and down cycle of motion
per second.
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What the speaker is doing at 1 Hz is the same at
any frequency. It is moving the air back and forth
in the direction that the sound wave propagates. If
you hold a piece of paper or a candle flame a few
inches in front of the speaker, you can see the paper
or flame being pushed back and forth as the sound
wave propagates away from the speaker.
If somebody drives past you in a car with a powerful
sound system with an enormous woofer cranked to the
max, you can sometimes see the windows in the car
move. (Imagine what's happening to their eardrums
inside the car!) Sometimes other loud, low frequency
sounds like explosions or music can be felt in your
chest. That's the pressure wave propagating from the
source, compressing the air in the direction of travel,
and compressing your chest as well!
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