Critical distance

Critical distance is, in audio physics, the distance at which the sound pressure level of the direct and the reverberant sound fields are equal when dealing with a directional source. In other words, the point in space where the amplitude of a reflected echo is the same as that of the source. This 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.

The calculation of the critical distance:

$d_c=\sqrt \frac{V}{100\pi\, RT_{60}} \approx \sqrt \frac{A}{50} \,$

or:

$d_c=\sqrt \frac{\gamma V}{100\pi\, RT_{60}} \approx \sqrt \frac{\gamma A}{50}\,$

γ is the degree of bundling of the source, V the room volume in cubic meters, RT₆₀ the reverberation time of room in seconds, and A the equivalent absorption surface.

Avoid the words Reverb Radius, Reverberation Radius (omnidirectional source), Hall Radius for Critical Distance.

This distance, called critical distance d_c, can be specified as a function of room volume and reverberation time using Sabine's approximation:

$d_c \approx 0.057 \cdot \sqrt \frac{V}{RT_{60}}$

Critical distance d_c in m
Room volume V in m³
Reverberation time RT₆₀ in s

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