<P> In acoustics, the sound pressure of a spherical wavefront radiating from a point source decreases by 50% as the distance r is doubled; measured in dB, the decrease is still 6.02 dB, since dB represents an intensity ratio . The pressure ratio (as opposed to power ratio) is not inverse - square, but is inverse - proportional (inverse distance law): </P> <Dl> <Dd> p ∝ 1 r (\ displaystyle p \ \ propto \ (\ frac (1) (r)) \,) </Dd> </Dl> <Dd> p ∝ 1 r (\ displaystyle p \ \ propto \ (\ frac (1) (r)) \,) </Dd> <P> The same is true for the component of particle velocity v (\ displaystyle v \,) that is in - phase with the instantaneous sound pressure p (\ displaystyle p \,): </P>

When does the inverse square law not apply