acoustics Acoustics is a branch of physics that deals with the study of mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician ...

, 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 In science, an inverse-square law is any scientific law stating that the observed "intensity" of a specified physical quantity is inversely proportional to the square of the distance from the source of that physical quantity. The fundamental ca ...

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 chamber An anechoic chamber (''an-echoic'' meaning "non-reflective" or "without echoes") is a room designed to stop reflection (physics), reflections or Echo (phenomenon), echoes of either sound or electromagnetic waves. They are also often isolate ...

s. 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. Simple source free field propogation

Equations

The

sound intensity Sound intensity, also known as acoustic intensity, is defined as the power carried by sound waves per unit area in a direction perpendicular to that area, also called the sound power density and the sound energy flux density. The SI unit of inte ...

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=\rho c I= \rho cW/4\pi r^2

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

L_p=L_w + 10\log_ (\rho c/ 400) - 10\log_ ( 4 \pi r ^2 )

In order to simplify this equation we can remove elements:

L_p=L_w  - 10\log_ ( 4 \pi r ^2 )

Measuring the

sound pressure Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrophon ...

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

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."

Characteristics

Examples and uses

Equations

References

See also