In
physics, an
inverse-square law is any
physical law stating that some physical
quantity or strength is
inversely proportional to the
square of the
distance from the source of that physical quantity.
In
acoustics, the
sound pressure of a
spherical wavefront radiating from a point source decreases by a factor of ½ as the distance
r is doubled, or measured in
dB it would decrease by 6.02 dB. The behaviour is not inverse-square, but is inverse-proportional:
However the same is also true for the component of
particle velocity that is
in-phase to the instantaneous sound pressure
.
Only in the
near field the
quadrature component of the particle velocity is 90° out of phase with the sound pressure and thus does not contribute to the time-averaged energy or the intensity of the sound. This quadrature component happens to be inverse-square. The
sound intensity is the product of the
RMS sound pressure and the RMS particle velocity (the in-phase component), both which are inverse-proportional, so the intensity follows an inverse-square behaviour as is also indicated above:
The inverse-square law pertained to sound intensity. Because sound pressures are more accessible to us, the same law can be called the "inverse-distance law".
.
