Specifications

Point Sources

The effects of geometric spreading are well known for the three idealised sound

sources; the point, line and plane. The behaviour of each is based solely on the

assumption that, in a homogeneous medium, sound propagation from a single point

source is purely spherical. Thus the sound energy in any particular direction is

inversely proportional to the increasing surface area of the sphere. If SWL represents

the continuous sound power output of the source measured at 1 metre, then at a

distance of r metres (where r must always be divided by the measurement distance,

which is usually 1m), the sound pressure level becomes;

SPL = SWL(point) - 10log (4

r²)

and I = W / (4

r²)

This is can be rewritten simply as;

SPL = SWL(point) - 20log (r) - 11

which is known as the standard inverse square law for point sources.

For a point source it represents a 6dB reduction in sound pressure level

per doubling of distance.

If the ground is quite hard and reflective, compensation must be made for these

ground reflections. In this case 11 is replaced by 8dB.

Line Sources (e.g. line array loudspeakers)

Classic line and plane sources can then be considered to consist of an infinite number

of evenly distributed individual point sources. The overall behaviour is then found by

integrating the individual effects of each point source over the full length or area. In

the case of an ideal line of infinite length, the results approximate that of purely

cylindrical propagation. Thus the sound energy in any perpendicular direction is

inversely proportional to the increasing circumference of the cylinder. Using the same

nomenclature as above, the sound pressure level becomes;

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