Now consider a point source emitting sound waves equally in all directions. From
everyday experience, we know that the intensity of sound decreases as we move farther
from the source. We identify an imaginary sphere of radius r centered on the source.
When a source emits sound equally in all directions, we describe the result as a spheri-
cal wave. The average power av emitted by the source must be distributed uniformly
over this spherical surface of area 4 r2. Hence, the wave intensity at a distance r from
the source is