Spatial Expansion
For a spherical sound source, the intensity in the radial direction as a function of distance r from the centre of the source is:
Here, Pac (upper case) is the sound power and A the surface area of a sphere of radius r. Thus the sound intensity decreases with 1/r2 the distance from an acoustic point source, while the sound pressure decreases only with 1/r from the distance from an acoustic point source after the 1/r-distance law.
= sound intensity at close distance
= sound intensity at far distance
Hence
where p (lower case) is the RMS sound pressure (acoustic pressure).
Read more about this topic: Sound Intensity
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