When a guitar's string is plucked, what causes the actual SOUND we
hear - the movement of the string, or the displacement of air caused
by the string? Would we be able to hear sound on the moon, where there is no air?
Plucking the string creates a sound wave in the air surrounding the string;
this sound wave is then communicated through the air in directions away from
the string, similar to the way a wave moves away from where a rock is tossed
into a pond. Note that the wave dissipates or weakens somewhat as it
travels from the source. Eventually the wave is depleted in the lake, and
similarly, the sound of a plucked guitar string is clearly not heard across
town unless there is adequate amplification. One can also use a telephone
or similar device which keeps the sound from being depleted over a distance.
The sound would not travel in the absence of a carrier. The string would
vibrate, but there would be nothing to carry the vibration to your ear. Try
tossing a rock into an empty lake bed. Do you notice any visible waves?
Without water in the lake, it is not likely.
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The vibrating guitar string sends a compression wave through the air, much
like a rock thrown into a pond sends a ripple outward. When the
compression wave reaches your eardrum, it causes the eardrum to move back
and forth, which your brain recognizes as a sound. The only way you would
be able to hear a sound on the moon is if it traveled through the ground
and was picked up either by your bones or the air in your space suit. In a
vacuum, the guitar string would vibrate longer than in air, because the air
won't carry away its vibrational energy. That doesn't gain you any
duration of tone, however, because it's precisely that transfer of energy
to the air that enables you to hear the sound in the first place.
Richard Barrans Jr., Ph.D.
The movement of the string causes it; the displacement of the air
conveys it. No.
Most of the sound travels through the air. However, if you were to attach one
end of the string to your skull you would 'hear' the sound as it was
transmitted through the bone directly from the string.
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Update: June 2012