Please help! What is the power supplied by the speaker?

Question

A speaker is placed between 2 observers, A and B, who are 121 m apart along a line connecting them. Observer A is 110 m from the speaker and observer B is 11 m from the speaker. If observer A records a sound level of 70 dB, and observer B records a sound level of 90 dB, find the power output supplied by the speaker.

Answer 1

ObA 110m, 70dB

ObB 11m 90dB,  behind the speaker

Only thing I can see to do is to convert the dB numbers to actual sound levels pressure. Then assume a sphere centered at the speaker and radius the distance to the observer, calculate it's area. With that area and the sound pressure in watts per sq meter, you can calculate the total pressure in watts.

A little unclear on the second observer behind the speaker. That may be additional power that you have to calculate the same way and add to the power off the front of the speaker.

Of course, this doesn't work unless the speaker and the observer are suspended in space with no other objects nearby.

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