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3.3.4 Energy Density

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Consider the case again of the oscillating piston in Figure 3.1. We shall consider the sound energy that is produced by the oscillating piston, as it flows along the tube from the piston source. We observe that the wavefront and the sound energy travel along the tube with velocity c metres/second. Thus after one second, a column of fluid of length c m contains all of the sound energy provided by the piston during the previous second. The total amount of energy E in this column equals the time‐averaged sound intensity multiplied by the cross‐sectional area S, which is from Eq. (3.9):

(3.16)

The sound energy per unit volume is known as the energy density ε,

(3.17)

This result in Eq. (3.17) can also be shown to be true for other acoustic fields as well, as long as the total sound pressure is used in Eq. (3.17), and provided the location is not very close to a sound source.

Engineering Acoustics

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