Critical Distance

Critical distance is one of the most practical concepts in sound system design because it defines the boundary between useful and problematic listening zones. Inside the critical distance, a listener hears predominantly direct sound from the loudspeaker, which carries clear, intelligible content. Outside the critical distance, the reverberant field dominates, smearing temporal detail and reducing speech intelligibility. The formula Dc = 0.057 × sqrt(Q × V / (π × RT60)) shows that critical distance increases with higher directivity (Q) and larger room volume, but decreases with longer reverberation time. This has immediate design implications: in a reverberant church with a 3-second RT60, the critical distance from an omnidirectional source might be only 2 metres. Switching to a line array with a Q of 20 extends critical distance dramatically, placing more of the audience in the direct-sound-dominant zone. For speech reinforcement, the rule of thumb is that listeners should be within 3× the critical distance for acceptable intelligibility. Beyond this, STI drops below the "fair" threshold of 0.45. This is why distributed speaker systems with multiple sources close to listeners outperform single-point systems in reverberant spaces. SonaVyx's RT60 measurement provides the reverberation time needed for critical distance calculation. The Room Scan tool estimates room volume. Together, these measurements let you calculate critical distance and plan speaker placement accordingly. AcousPlan's RT60 calculator provides complementary design-time predictions before the system is installed.