The Critical Distance is the distance measured from the sound source where direct sound and reflected / reverberant sound are equal in intensity (SPL).
The Critical Distance is depending on the characteristics of the room and the characteristics of the sound source. The more narrow the 'beam' of a loudspeaker the longer is the critical distance. A more reverberant room creates a shorter and an acoustically dry room creates a longer critical distance.
|critical distance for an omnidirectional source||critical distance for any source with a directivity factor Q|
dc: critical distance in meter
V: room volume in cubic meter
RT: reverberation time in seconds
How to find critical distance approximately:
Moving away from a sound source with a sound pressure meter, there is one certain position where the level will not diminish any more. At this point direct sound energy and reverberant sound energy are equal.
The critical distance resulting from an omnidirectional sound source (Q=1) is the Reverberation Radius. The reverberation radius is a parameter depending only on the room characteristics and the critical distance room is a parameter depending on sound source directivity and room characteristics.
Any loudspeaker or any other non-omnidirectional source (like human voice) will cause a different critical distance in a room depending on its directivity.
Microphones for speech reinforcement or recording should always be placed within critical distance of the talking person. Otherwise the signal will sound very reverberant or, in the case of reinforcement, will be prone to feedback. To get satisfactory results omnidirectional microphones should be positioned no farther from the talking person than 30 percent of Dc, unidirectional microphones (cardioid, supercardioid, or shotgun) should be positioned no farther than 50 percent of Dc.