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Sound is simply an airborne version of vibration. The air which carries sound is a mixture
of gases. In gases, the molecules contain so much energy that they break free from
their neighbors and rush around at high speed. As Figure 1.1(a) shows, the innumerable
elastic collisions of these high-speed molecules produce pressure on the walls of any
gas container. If left undisturbed in a container at a constant temperature, eventually the
pressure throughout would be constant and uniform.
