posted by unknown .
A stereo speaker represented by P in Figure 13-22 emits sound waves with a power output of 130.0 W. What is the intensity of the sound waves at point x when r = 7.8 m? W/m2
Divide the power by 4 pi r^2 to get then power per area (intensity) at distance r. I get 0.17 W/m^2
Then convert intensity to decibels with the usual logarithmic formula. It's pretty loud ... over 110 dB.
This assumes that the speaker radiates sound uniformly in all directions. Few speakers work that way.