Measurement:Inverse Square Law

Inverse Square Law

If we double the radius of the sphere to 0.564 m, the surface area of the sphere quadruples because the radius is squared in the area equation (A = 4πr2). Thus our intensity will drop to one-fourth its former value. (Note, however, that the total acoustic power is still 1 W so the LW still is 120 dB.) Now an intensity change from 1 W to 0.25 W/m2 can be written as a decibel change. The acoustic intensity (i.e., the power per unit of area) has dropped 6 dB in any given area:

Measurement-0046

Therefore our LP had to also drop 6 dB and would now be approximately 114 dB.

This effect is commonly called the inverse square law change in level. Gravity, light, and many other physical effects exhibit this rate of change with varying distance from a source. Obviously, if you halve the radius, the levels all rise by 6 dB.


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Ahmed Farahat is EECS engineer With 18 years of experience in the field he worked on different technological discipline and and had honored Post Graduate Diploma In Computer Science And Engineering

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