A sound wave emitted by a compact
source is a spherical outgoing wave, and the amplitude of the
wave falls off like *1/r,* so that we can write

Therefore the intensity *I,* in
Watts per square meter, goes like the average source power *P*
divided by the area subtended at a distance *r* from the
source, *4πr ^{2}*

travelling wave... |
Standing Wave... |