How does the Ear Guitar work?
When you pluck the string on an Ear Guitar, the string starts vibrating.
The vibration in the string starts the bottom of the cup vibrating, which
starts the air inside the cup vibrating. The cup helps channel those vibrating
air molecules into your ear-so you hear the sound loud and clear.
Your voice, like other sounds, is a vibration. (Put your hand on your
throat as you talk and you'll feel the vibrations.) When you talk into one
of the cups, the vibrations of your voice travel into the cup, then from
the cup into the string, and then back into the other cup. The cup channels
your voice into your friend's ear.
Your yogurt-cup telephone works if the string between the two cups can
vibrate freely. Pinch the string between the two cups, and your friend won't
hear your voice as well. You may also discover that the string between the
cups must be pulled tight, or your telephone won't work. If the string is
loose, the sound vibrations die out before they reach the other cup.