Critical distance is, in
acoustics, the distance at which the
sound pressure level of the direct sound D and the
reverberant sound R are equal when dealing with a directional source. As the source is directional, the sound pressure as a function of distance between source and sampling point (listener) varies with their relative position, so that for a particular room and source the set of points where direct and reverberant sound pressure are equal constitutes a surface rather than a distinguished location in the room. In other words, it is the point in space at which the combined
amplitude
The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...
of all the reflected echoes are the same as the amplitude of the sound coming directly from the source (D = R). This distance, called the critical distance
, is dependent on the geometry and
absorption of the space in which the sound waves propagate, as well as the dimensions and shape of the sound source.
A reverberant room generates a short critical distance and an acoustically dead (
anechoic) room generates a longer critical distance.
Calculation
The calculation of the critical distance for a diffuse approximation of the reverberant field:
:
where
is the degree of directivity of the source (
for an omnidirectional source),
the equivalent absorption surface,
the room volume in m
3 and
the
reverberation time
Reverberation (also known as reverb), in acoustics, is a persistence of sound, after a sound is produced. Reverberation is created when a sound or signal is reflected causing numerous reflections to build up and then decay as the sound is abso ...
of room in seconds. The latter approximation is using Sabine's reverberation formula
.
Sources
Acoustics
Audio effects
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