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Sound is a longitudinal wave in some material medium, usually in air. Alternatively, we can say that sound is a wave of pressure variation, as Figure 12-1 shows.

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Figure 12-1

Notice several things about the above graphs. The variations of pressure are much smaller than the barometric pressure itself, that is, the equilibrium pressure Peq. In fact, the pressure variations are much exaggerated in the figure. The quantity Δx gives the displacement of an air particle from its equilibrium position to its position with the sound wave, so that a positive Δx corresponds to displacement to the right. Where Δx = 0, the pressure is a maximum or minimum, and where P = Peq, we have Δx a maximum or minimum. We denote the variation of the pressure from the equilibrium pressure as ΔP = PPeq.

There are many ways in which energy can be converted into the energy of sound; for example, an underground nuclear explosion creates pressure waves in the solid Earth. The vibrating column of air in the plaintive oboe creates pressure waves in air.

In general, sound waves travel faster in a stiff material than in a material which is not as stiff. Thus waves travel a little faster in solids than in liquids, and a lot faster in solids and liquids than in a gas. (See table.) material speed of sound
material speed of sound
air 340 m/s
water 1600 m/s
steel 16000 m/s

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