# Answer on Mechanics | Relativity Question for adam

Question #2276

A longitudinal wave spreads radially from a source with power 345 W. What is the intensity 40.0 meters away? answer in W/m^2.

Expert's answer

For a spherical sound source, the intensity in the radial direction as a function of distance r from the centre of the source is:

Here, P

Thus the sound intensity decreases with 1/r

So we have

**I**_{r}=P_{ac}/(4πr^{2})Here, P

_{ac}=345 W (upper case) is the sound power and A the surface area of a sphere of radius r.Thus the sound intensity decreases with 1/r

^{2}the distance from an acoustic point source, while the sound pressure decreases only with 1/r from the distance from an acoustic point source after the 1/r - distance law.So we have

**I**_{r}=P_{ac}/(4πr^{2})=345/(4π 40^{2})=345/(4*π*1600)=0.017 W/m^{2}.Need a fast expert's response?

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