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Propagation in Free Field

As implied by its name, an acoustic free field is a situation where there are no obstacles close to the sound source, the receiver, and the sound path. Under such conditions the sound waves from an omnidirectional point sound source will be spherical, and the sound pressure level Lp as a function of the sound power level Lw of the source and the distance d from source to receptor will be expressed as

where Q is the directivity of the source:

Q = 1 for an omnidirectional source

Q = 2 for an omnidirectional source over a reflective plane Q = 4 for an omnidirectional source in a corner

D is the extra attenuation (e.g., due to barriers, ground effects, etc.).

As other kind of sources can be obtained by adding sets of point sources over a given line or surface, one may note that the spatial sound level decrease with the distance varies as:

  • 20 log(distance) for a point source
  • 10 log(distance) for a line source of length L, as long as d < L
  • 10 log(distance) for a surface source of maximum dimension L, as long as d < L

When dealing with a large source, it is always possible to divide it into smaller elements of area S whose dimensions will be such that the distance from the source to the point of calculation will be much larger than square(S) in order to try to assimilate it to a point source. Further away, the spatial sound level decrease with the distance varies as 20 log(distance).

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