Sound Field Terms, Definitions, Units, Measurements ...
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.
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 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
Free Field Level
is the sound pressure level
, away from 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 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
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.
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.
the scattering or random reflection of sound waves
. A diffuser changes the reflected sound so listeners may perceive the sound coming from all directions.