Free field (acoustics)

For the concept in physics, see Free field.

In acoustics, a free field is a situation or space in which no sound reflections occur. ^{[1] [2]}

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. ^[3] In a free field, sound is attenuated with increased distance according to the inverse-square law. ^[1]

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. ^[1]

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. ^[1]

${\displaystyle p^{2}=\rho cI=\rho cW/4\pi r^{2}}$ ^[1]

To calculate for air pressure, the equation can be written differently: ^[1]

${\displaystyle L_{p}=L_{w}+10\log _{10}(\rho c/400)-10\log _{10}(4\pi r^{2})}$ ^[1]

In order to simplify this equation we can remove elements: ^[1]

${\displaystyle L_{p}=L_{w}-10\log _{10}(4\pi r^{2})}$ ^[1]

Measuring the sound pressure level at a reference distance (Rm) from the source allows us measure another distance (r) more easily than other methods: ^[1]

${\displaystyle {\displaystyle L_{p}=L_{m}-20\log _{10}(r/r_{m})}}$ ^[1]

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." ^[1]

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

• Line array § Theory