Critical Distance

Critical distance is the distance from a sound source at which the direct sound level equals the reverberant sound level. Beyond this distance, increasing separation provides negligible noise reduction because the reverberant field dominates. Critical distance depends on the room constant (a function of room absorption) and the source directivity factor. In typical untreated offices, critical distance is only 1–2 m, meaning most listeners are in the reverberant field. Increasing room absorption extends the critical distance, improving direct-to-reverberant ratio and speech intelligibility. For a source with directivity factor Q in a room with room constant R, critical distance is approximately 0.14√(QR). Critical distance is fundamental to PA system design (determining maximum listener distance), open-plan office acoustics (speech privacy through reverberant field control), and noise control (determining whether adding absorption will reduce levels at worker positions).