Acoustic Glossary


 

Sound Fields : Definitions, Terms, Units, Measurements..

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, absorbed or diffused by.

Sound Fields come in various forms : Active, Anechoic, Diffused, Direct, Far, Free, Near, Reactive, Reverberant.

Sound Field IEC Definition, region of an elastic medium containing sound waves


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 sound 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 see anechoic chamber.


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
Diffraction Factor


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

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 IEC Definition, 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 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

Diffuse Field Sensitivity IEC Definition, 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


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.

Far Sound Field IEC Definition, sound field distant from a sound source where instantaneous sound pressure and particle velocity are substantially in phase


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.

Free Sound Field IEC Definition, sound field in a homogeneous isotropic medium whose boundaries exert a negligible effect on the sound waves

Free Field Room a room whose walls, ceiling, and floor are lined with sound-absorbing materials to minimise all sound reflections

Free Field Room IEC Definition, room whose boundaries absorb substantially all the sound incident thereon, thereby affording free-field conditions

See also Active FieldAnechoic and Anechoic Chamber

Near Sound Field IEC Definition, sound field near a sound source where instantaneous sound pressure and particle velocity are substantially out of phase

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 FieldStanding Wave


Reverberant Sound Field IEC Definition, sound field in which substantially all sound waves have been reflected several times from a boundary of the medium

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

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