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Wave, Sound, and Music


Sound Wave A compressional wave in air can be set up by the back-and-forth motion of a speaker as shown in Figure 09. Here, the air molecules are alternately pressed together and pulled apart by the action of the speaker. The result is a propagating wave in which the pressure (and density) of the air varies with distance in a regular way - the pattern is, in fact, exactly the same as the displacement pattern of a transverse wave on a string (see Figure 01 and 02). Compressional waves in air are called sound waves, which are always longitudinal waves with the vibration parallel to the direction of propagation. Most of the previously mentioned concept about waves can be applied to the sound wave without modification except the formula for the wave velocity in Eq. (3) where the tension is replaced by the "bulk modulus" (change in pressure / change in volume) and the linear density is just the density of the air. It turns out that the velocity of sound at STP is about 330 m/s.

Figure 09 Sound Wave
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