Free field (acoustics)

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Free field, in acoustics, is a situation or space in which no sound reflections occur because the sound is unobstructed. ^{[1] [2]}

Characteristics[]

The lack of any 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]

As the sound moves through the free field it spreads evenly and gets quieter the farther it travels. ^[1]

Examples and uses[]

In nature, free field conditions can only be found if sound reflections on the floor do not matter, e.g. in new snow in a field or approximately at good sound-absorbing floors (deciduous, dry sand, etcetera.)

Free field conditions can be artificially produced in anechoic chambers. In particular free field conditions play a major role in the field of acoustic measurements and sound perception experiments, since the results are influenced only by the sound of the sound source and not from reflections of the room.

Even 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 simplest example possible shown in Figure 1, a singular sound source emits sound evenly and spherically with no obstructions. ^[1]

Figure 1.

Equations[edit][]

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 6db 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]

Reference[]

See also[]

• Line array#Theory