Acoustic Glossary


Sound Fields : Definitions, Terms, Units and Measurements

Sound Fields come in various forms for example Active Fields, Direct Fields, Diffused Fields, Far Fields, Free Fields, Near Fields, Reactive Fields, Reverberant Fields. This web page explains and defines the differences.

Sound Fields depend not only on the Sound Power and directional characteristics of the Sound Source, but also on the properties of any medium it passes through, is reflected by, absorbed or diffused by.


Active Field : a sound field in which the Particle Velocity and the Sound Pressure are in Phase. All the acoustic energy is transmitted; none is stored and the product of the pressure and the particle velocity = the net Sound Intensity.

A Plane Wave propagating in Free Field is an example of a purely active sound field.


Anechoic : without echo - refers to the absence of sound reflections. It is almost impossible to create a truly anechoic environment, as there is no such thing as a perfect Sound Absorber.


Critical Distance : the distance at which the sound pressure level of the direct sound and the reverberant sound are equal when dealing with a directional source.


Diffraction : phenomenon by which a sound wave is changed in direction by an obstacle or other heterogeneity in the medium.


Diffuse Field : the region in a room where the Sound Pressure Level is uniform i.e. the reflected sound dominates, as opposed to the region close to a noise source where the direct sound dominates. The same as Reverberant Field.

In a diffuse field the Sound Pressure and the Particle Velocity phase varies randomly so the net Sound Intensity is zero.

Sound that is completely random in phase; sound that appears to have no single source.

Diffuse Field Distance : distance from the sound source at which the sound pressure of the direct sound is equal to the sound pressure of the reverberant sound in the room containing the source


Diffusion : the scattering or random reflection of a sound wave from a surface. A diffuser changes the reflected sound so listeners may perceive the sound coming from all directions.


Direct Field : the region in which the sound measured can be attributed to the source alone without reflections. Early reflections that reach the listener within 50 ms integrate with the direct sound and can improve speech clarity. Later reflections may have a negative effect on speech clarity


Far Field : a region in free space, distant from a sound source, where the Sound Pressure Level obeys the Inverse Square Law (the sound pressure level decreases 6 dB with each doubling of distance from the source). Also, in this region the Sound Particle Velocity is in phase with the Sound Pressure.


Free Field : a sound field region with no adjacent reflecting surfaces. In practice a free-field can be said to exist if the Direct sound is 6 dB or preferably 10 dB greater than the Reverberant or reflected sound.

see also
Active Field
Anechoic Chamber

Near Field : that part of a sound field, usually within about two wavelengths of a noise source, where there is no simple relationship between sound level and distance, where the sound pressure does not obey the Inverse Square Law and the Particle Velocity is not in phase with the Sound Pressure.


Reactive Field : a sound field in which the Particle Velocity is 90° out of phase with the pressure, therefore the Sound Intensity is zero.

see also : Active Field and Standing Wave

Reverberant Field : the region in a 'room' where the reflected sound dominates, as opposed to the region close to the noise source where the direct sound dominates.

see also : Diffused Field