Physics - Creating sound waves
Creating sound waves
As noted, sound is a mechanical wave, and a longitudinal wave. It involves
the vibration of the molecules within the medium parallel to the direction
of travel of the sound wave itself. It involves the transfer of energy but
no net displacement of the matter carrying that energy. In the diagram
below you can see how a sound wave is created. All sound waves must have a
vibrating sound source. Examples include the strings vibrating as a violin
is bowed, the skin of a drum vibrating once it has been struck, the vocal
chords within the larynx vibrating as somebody speaks.
Drum vibration video
The vibrating sound source initially moves forward and the air molecules
nearby are compressed; we say an area of compression has been created.
As the sound source moves backwards, it leaves a volume more empty of air
molecules than usual. That is, a rarefaction  or area of low air
pressure is formed. This process is continued over and over so that a
sequence of compressions and rarefactions are formed as shown. These
compressions and rarefactions travel away from the source.
Any individual air molecule will actually just vibrate rapidly to and fro,
around a central median position as the compressions and rarefactions
travel away from the source. The air molecules do not undergo any overall
change to their mean position. As the sound energy travels through a region
of space any individual air molecule will simply vibrate.
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