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Free field (acoustics)

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For the concept in physics, see Free field.
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In acoustics, a free field is a situation or space in which no sound reflections occur.

Characteristics

The lack of reflections in a free field means that any sound in the field is entirely determined by a listener or microphone because it is received through the direct sound of the sound source. This makes the open field a direct sound field. In a free field, sound is attenuated with increased distance according to the inverse-square law.

Examples and uses

In nature, free field conditions occur only when sound reflections from the floor can be ignored, e.g. in new snow in a field, or approximately at good sound-absorbing floors (deciduous, dry sand, etc.) Free field conditions can be artificially produced in anechoic chambers. In particular, free field conditions play a major role in acoustic measurements and sound perception experiments as results are isolated from room reflections.

With voice and sound recordings, one often seeks a condition free from sound reflections similar to a free field, even when during post-processing specifically desired spatial impression will be added, because this is not distorted by any sound reflections of the recording room.

In the simple example shown in Figure 1, a singular sound source emits sound evenly and spherically with no obstructions.

Figure 1.

Equations

The sound intensity and pressure level of any point in a free field is calculated below, where r (in meters) is the distance from the source and "where ρ and c are the air density and speed of sound respectively.

p 2 = ρ c I = ρ c W / 4 π r 2 {\displaystyle p^{2}=\rho cI=\rho cW/4\pi r^{2}}

To calculate for air pressure, the equation can be written differently:

L p = L w + 10 log 10 ⁡ ( ρ c / 400 ) − 10 log 10 ⁡ ( 4 π r 2 ) {\displaystyle L_{p}=L_{w}+10\log _{10}(\rho c/400)-10\log _{10}(4\pi r^{2})}

In order to simplify this equation we can remove elements:

L p = L w − 10 log 10 ⁡ ( 4 π r 2 ) {\displaystyle L_{p}=L_{w}-10\log _{10}(4\pi r^{2})}

Measuring the sound pressure level at a reference distance (Rm) from the source allows us measure another distance (r) more easily than other methods:

L p = L m − 20 log 10 ⁡ ( r / r m ) {\displaystyle {\displaystyle L_{p}=L_{m}-20\log _{10}(r/r_{m})}}

This means that as the distance from the sources doubles, the noise level decreases by 6 dB for each doubling. However if the sound field is not truly free of reflections, a directivity factor Q will help "characterise the directional sound radiation properties of a source."

References

  1. ^ Hansen, Colin (January 1951). "FUNDAMENTALS OF ACOUSTICS" (PDF). American Journal of Physics.
  2. Ray, Elden (16 June 2010). "INDUSTRIAL NOISE SERIES Part IV MODELING SOUND PROPAGATION" (PDF).
  3. "sound fields - acoustic glossary". www.acoustic-glossary.co.uk. Retrieved 2021-05-16.

See also

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