Acoustic Glossary


Sound Fields Terms and Definitions .....

Sound Fields are subject to the acoustic properties of any media the sound waves pass through, i.e. are reflected, absorbed or diffused by.

Sound Fields are root power quantities and not power quantities

Sound Field Definition (IEC 801-23-27) region of an elastic medium containing sound waves

Active Sound Fields occur when the sound pressure and the particle velocity are in phase and all the acoustic energy is transmitted, none is stored and the sound intensity = sound pressure x particle velocity.

A plane wave propagating in free sound field is an example of a purely active sound field.

Anechoic Room under free-field room

Critical Distance see diffuse field distance

Diffraction Factor

Diffuse Field Distance is where the sound pressure of the direct sound is equal to the sound pressure of the reverberant sound in the room containing the source. Also known as the critical distance.

Diffuse Field Distance Definition (IEC 801-31-17) that distance from the acoustic centre of a sound source at which the mean-square sound pressure of the direct sound, in a specified direction, is equal to the mean-square sound pressure of the reverberant sound in the room containing the source

Diffuse Sound Field is 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.

In a diffuse sound field, the sound pressure and the particle velocity are not in phase so the net sound intensity is zero and the sound doesn't appear to have a single source.

Diffuse Sound Field Definition (IEC 801-23-31) sound field which in a given region has statistically uniform energy density, for which the directions of propagation at any point are randomly distributed.

Direct Sound is 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. However later reflections may have a negative effect on the clarity

Far Sound Field is the region, some distance from the sound source, where the sound level conforms with the inverse square law i.e. the sound level decreases by 6 dB for each doubling of the distance from the sound source.

Far Sound Field Definition (IEC 801-23-30) sound field distant from a sound source where the instantaneous sound pressure and particle velocity are substantially in phase

Free-field Room, a room whose walls, ceiling, and floor are lined with sound-absorbing materials to minimise all sound reflections, also known as an anechoic room

Free-field Room Definition (IEC 801-31-18) room whose boundaries absorb substantially all the sound incident thereon, thereby affording free-field conditions

See also anechoic chamber

Free Field Sound Level is the sound pressure level, well away from any reflecting surfaces

Measurements made 1.2 m to 1.5 m above the ground and at least 3.5 m away from other reflecting surfaces are usually regarded as free-field

Free Sound Fields occur where there are no nearby reflecting surfaces. In practice a free sound field can be said to exist if the direct sound level is 6 dB higher than the reverberant or reflected sound level.

Free Sound Field Definition (IEC 801-23-28) sound field in a homogeneous isotropic, medium whose boundaries exert a negligible effect on the sound waves

See also, active fieldanechoicboundary effect

Near 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.

Near Sound Field Definition (IEC 801-23-29) sound field near a sound source where instantaneous sound pressure and particle velocity are substantially out of phase

Reactive Sound Field, a sound field in which the particle velocity is 90° out of phase with the sound pressure and therefore the sound intensity is zero.

See also, active sound fieldstanding wave

Reverberant Sound 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.

Reverberant Sound Field Definition (IEC 801-23-32) sound field in which substantially all sound waves have been reflected several times from a boundary of the medium

See also, diffused fieldreverberation

Sound Diffusion in rooms is due to the scattering and the random reflections of the sound energy by the walls and other objects in the room. Not usually a serious problem at home but in larger areas like studios, schools and concert halls the resulting sound levels may not be uniform, throughout the space, due to low frequency room modes in these larger spaces.

Sound Diffusers are commercially available to diffuse specific reflections, room modes and other acoustic anomalies

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