Room Critical Distance Calculator
In any reverberant room there is a distance at which the direct sound from a source and the built-up reverberant field are equally loud. This critical distance, or reverberation radius, marks the boundary between the near zone where moving closer keeps improving clarity and the far zone where reverberation takes over. This calculator estimates the critical distance from room volume, reverberation time, and the source directivity factor Q. Use it to plan loudspeaker coverage, microphone placement, and seating, and to see how a more directional source pushes the critical distance further out.
Critical distance formula
Dc = 0.057 * sqrt(Q * V / RT60)
V = room volume (m3)
RT60 = reverberation time (s)
Q = source directivity factor (1 omni, 2 floor, 8 corner)
The critical distance grows with the square root of volume and source directivity and shrinks as reverberation time increases. A more directional source (higher Q) projects useful direct sound further into the room.
Worked example
A 500 m3 room with RT60 = 1.2 s and Q = 2 gives Dc = 0.057 * sqrt(2 * 500 / 1.2) = 0.057 * sqrt(833.33) = 1.65 m. With an omnidirectional source (Q = 1) the critical distance falls to 1.16 m.
Critical distance: frequently asked questions
What is critical distance?
The critical distance, also called the reverberation radius, is the distance from a sound source at which the direct sound and the reverberant sound have equal level. Closer than this, the listener hears mostly direct sound; further away, the diffuse reverberant field dominates and intelligibility no longer improves with proximity in the same way.
What is the critical distance formula?
A widely used estimate is Dc = 0.057 times the square root of (Q times V divided by RT60) in metric units, where Q is the source directivity factor, V is room volume in cubic metres, and RT60 is reverberation time in seconds. The 0.057 constant follows from the room-constant relationship and the Sabine equation.
What is the directivity factor Q?
Q describes how a source concentrates its output. An omnidirectional source has Q = 1; a source on a hard floor radiating into a hemisphere has Q = 2; a source in a corner has Q = 8. A directional loudspeaker pointed at the audience can have a much higher Q. It is a user-editable input here.
Why does critical distance matter?
It tells you how far you can place listeners or microphones before reverberation dominates and clarity falls. In sound reinforcement, keeping listeners within the critical distance (or raising it with directional speakers and absorption) is central to good intelligibility.
Official sources
- International Organization for Standardization: ISO 3382 room acoustic parameters.
- U.S. National Institute of Standards and Technology: nist.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.