Source Distance and DRR

From a single recording, libsonare estimates how far the source was from the microphone — though not which direction. The chain that gets there runs from the direct-to-reverberant ratio, through the room's critical distance, to a distance in meters.

DRR — direct-to-reverberant ratio

The direct-to-reverberant ratio (DRR) is the level of the direct sound — the first wavefront to reach the mic, straight from the source — relative to the reverberant energy that follows, expressed in decibels.

DRR = 10·log₁₀( direct energy / reverberant energy )

DRR is the single strongest cue for distance. Move the mic closer and the direct sound gets louder while the room's reverberant field stays roughly constant, so DRR rises. Back away and the direct sound falls toward the steady reverberant level, so DRR drops.

• High DRR — close, dry, present; the source is "in your face."
• Low DRR — distant, washy; the room dominates the sound.

This is exactly the cue your ears use to judge distance in a familiar room, which is why a close-mic'd vocal sounds intimate and a far one sounds like it is "across the room."

Critical distance

The critical distance is the distance from the source at which the direct sound and the reverberant field are equally loud — where DRR equals 0 dB.

Closer than the critical distance, the direct sound wins and the source reads as clear and localized. Farther, the reverberant field wins and the sound becomes diffuse and room-dominated. Critical distance depends on the room: a dead room (high absorption, short RT60) pushes it far out, so you can stand well back and still hear a direct sound; a live room pulls it close, so even a few steps back drowns the source in reverberation.

Roughly, the more absorptive the room, the weaker its reverberant field, so the direct sound stays dominant farther out — critical distance grows with the square root of the room's total absorption (the A in Sabine's equation: surface area weighted by how absorptive each surface is). That is why the same talker sounds intelligible across a treated studio but muddy across a cathedral at the same distance.

Source distance

Putting them together: the measured DRR, referenced against the room's critical distance, places the source at an estimated distance in meters from the listener/mic. A DRR of 0 dB lands the source right at the critical distance; higher DRR pulls it closer, lower pushes it farther.

The hard limit is geometry. A single channel carries one number — energy versus time — so it can resolve one equivalent distance but contains no directional information. The scanner therefore draws the estimated source as a full shell (a sphere) around the listener at the estimated radius, not as a point: every direction at that distance is equally consistent with what one mic heard. Recovering direction would need at least two channels and the time/level differences between them.

A further consequence: multiple real sources — several talkers, a whole band — collapse into a single equivalent distance, because one channel cannot separate them.

How libsonare estimates distance

libsonare separates the direct sound from the reverberant tail in the (possibly blindly recovered) impulse response, forms the DRR from their energy ratio, and estimates the room's critical distance from the recovered volume and absorption. The source distance follows from the DRR-versus-critical-distance relationship of the diffuse-field model. Because the model is single-channel, the result is a radius, not a position, and the visualization renders it as a distance shell around the listener. Confidence on the distance estimate tracks how cleanly the direct sound could be separated from the tail — sources well past the critical distance, where direct energy is faint, are inherently harder to place.

Related: Reverberation Time (RT60 and EDT), Room Geometry and Volume, Inverse Room Estimation, Acoustic Analysis