16.7.
Sound Intensity
The sound intensity I is defined as the power P that passes perpendicularly through a surface divided by the area A of the surface:
 | (16.8) |  |
The unit of sound intensity is W/m2. If the source emits sound uniformly in all directions, the radiated power P passes through the spherical surface area A = 4pr2, where r is the distance to the sound source. The intensity is:
 | (16.9) | |
Example 11
At a distance of 3.8 m from a siren, the sound intensity is 3.6 × 10-2 W/m2. Assuming that the siren radiates sound uniformly in all directions, find the total power radiated.
Using equation (16.9) and solving for the power P gives
| P = 4pr2I = 4p(3.8 m)2(3.6 × 10- 2 W/m2) = 6.5 W | |
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