DEMO: You can use a
Slinky to get a good idea of how compressional waves, such as
sound, move. Stretch a Slinky out and then give one end a good
strong knock or jiggle. This will push the coils near your hand
into the ones next to them, which will be pushed into the next
and the next and so on, all the way to the other end. At the
other end, the compression will rebound and move back in the
other direction. Thus a "compression" moves back and
forth, comparable to a sound wave and an echo. In sound waves, it
is molecules of air that get pushed into each other, rather than
coils of a spring, but it is the same concept. Of course, sound
will also travel through water and even solid materials (how else
could you "hear through walls"?). Even in this case,
the sound waves move by pushing molecules forward in the
direction of motion.
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