# More sound wave help

1. Dec 11, 2004

### tmkgemini

I'm really confused on this one too..

The intensity of the sound from a certain source is measured at two points along a line from the source. The points are separated by 13.1 m, the sound level is 71.30 dB at the first point and 63.20 dB at the second point. How far is the source from the first point? What is the power output of the source?

I've looked all through my notes and the book and i can't find an equation to help

2. Dec 12, 2004

### Andrew Mason

I think you would use the inverse square law for sound - it propagates in all directions with wave fronts on the surface of an expanding sphere. There can be variation across that surface where the sound is directional, but those variations still propagate as 1/r^2. Keep in mind that it is the power/area that obeys the 1/r^2 law, not loudness (decibels). So you have to find the relative intensities at those two points and relate them with the 1/r^2 law: ie. $I_1/I_2 = r_2^2/r_1^2$

The Power Output of the source (assuming a uniform sound in all directions) would be the intensity in watts/m^2 at I_1 say, multiplied by the area of a sphere of radius D_1 where D_1 is the distance from the source of the sound.

AM

Last edited: Dec 12, 2004