Acoustic Critical Distance Calculator
The critical distance is where a source's direct sound and the room's reverberant sound are equally loud. Inside it, speech and music stay clear because direct sound leads; beyond it, reflections dominate and intelligibility drops. This calculator uses the standard relation linking critical distance to source directivity, room volume, and reverberation time. Enter the directivity factor Q (1 for omnidirectional, higher for directional or boundary-mounted sources), the room volume, and the RT60.
Critical distance formula
Dc = 0.057 * sqrt(Q * volume / RT60)
Dc (ft) = Dc * 3.28084
omnidirectional Dc = 0.057 * sqrt(volume / RT60)
directivity gain = sqrt(Q)
Critical distance grows with the square root of source directivity and room volume, and shrinks with the square root of reverberation time. The 0.057 constant is for metric units (volume in cubic metres, RT60 in seconds).
Critical distance facts
- Q = 1 is omnidirectional; a flush wall gives 2, a wall-floor edge 4, a corner 8.
- Doubling Q multiplies the critical distance by the square root of 2.
- A more reverberant room (higher RT60) shrinks the critical distance.
- Keeping listeners within Dc preserves speech intelligibility.
- Large, live rooms often need directional or distributed loudspeakers.
Critical distance: frequently asked questions
What is critical distance in acoustics?
Critical distance is the distance from a sound source at which the direct sound level equals the reverberant (reflected) sound level. Closer than this, direct sound dominates and speech is clearer; farther away, the reverberant field dominates.
How is critical distance calculated?
A common engineering form is Dc = 0.057 times the square root of (directivity factor Q times room volume divided by RT60), in metres. It rises with source directivity and room volume and falls as reverberation time grows.
What is the directivity factor Q?
Q describes how much a source focuses energy forward. An omnidirectional source has Q = 1; mounting it against a wall doubles Q to 2, a wall-floor junction gives 4, and a corner gives 8. A directional loudspeaker can have a higher on-axis Q.
Why does critical distance matter for sound systems?
Placing listeners within the critical distance keeps speech clear because direct sound dominates. In large reverberant rooms the critical distance is short, which is why distributed or directional speakers are used to keep audiences in the direct field.
Where do RT60 and volume come from?
RT60 is the room reverberation time, which you can measure or estimate with the Sabine equation, and volume is the room volume in cubic metres. Both are user inputs here because they depend on your specific room.
Official sources
- International Organization for Standardization: ISO 3382 acoustics, reverberation time.
- Acoustical Society of America: room acoustics references.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.