Acoustic Glossary


 

Sound Field Terms, Definitions, Units, Measurements ...


Sound Fields depend on the sound power of the sound source and the acoustic properties of any medium the sound waves pass through, i.e. reflected, absorbed and diffused by.

Sound Fields are classified as 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 field is an example of a purely active sound field.


Anechoic

Critical Distance in acoustics, is the distance from the sound source where the direct sound level and the reverberant sound level are equal, also known as the direct sound distance.

At the critical distance the measured sound level will be 3 dB higher than the direct and reverberant sound levels, i.e. the sum of the two levels.


Diffraction
Diffraction Factor

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 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. See also the critical distance above.

 

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 Field Sensitivity Definition IEC 801-25-65, of an electroacoustic transducer for sound reception, at a specified location, for a specified frequency, quotient of the root mean square open circuit output voltage due to sound waves arriving more or less simultaneously with equal probability from all directions, by the root mean square sound pressure at that location due to the same sound waves but in the absence of the electroacoustic transducer

 

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 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 Sound Field is the region some distance from the sound source, where the sound level obeys the inverse square law i.e. the sound level decreases by 6 dB for each doubling of the distance from the source. Also, in this region the sound particle velocity is in phase with the sound pressure.

 

Far Sound Field Definition IEC 801-23-30, sound field distant from a sound source where 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 Sound Field, a sound field region with no adjacent reflecting surfaces. In practice a free sound field can be said to exist if the direct sound is 6 dB or preferably 10 dB greater than the reverberant or reflected sound.

 

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, 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.

 

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 pressure, 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, the scattering or random reflection of sound waves. A diffuser changes the reflected sound so listeners may perceive the sound coming from all directions.

 

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