When a point source (or any source
that radiates equally in all directions) radiates into free space, the intensity of the
sound varies as where r is the distance from the source.
This may be understood as a given amount of sound power being distributed over the surface
of an expanding sphere with area 4p r2 (see Fig. 5). Thus
the intensity is given by :
(15)
Figure 5 Spherical Sound Waves in
a Free Field
where W is the power of the source. An
environment in which there are no reflections is called a free field. In a
free field, the sound intensity level decreases by 6 dB each time the distance from the
source is doubled. The sound intensity level (or sound pressure level) at a distance of
one meter from a source in a free field is 11 dB less than the sound power level of the
source. This is easily shown as follows:
(16)
Similarly, it can be shown that at a
distance of two meters, Liis 17 dB less than Lw.
where r is the distance from the source.
This may be understood as a given amount of sound power being distributed over the surface
of an expanding sphere with area 4p r2 (see Fig. 5). Thus
the intensity is given by :
(15)
(16)