Reverberation ratio is the primary cue for judging source distance, and directional cues cannot substitute for it

What it is

People judge how far away a sound source is mainly not through directional cues (interaural time difference, interaural level difference, pinna filtering) but through the ratio of direct sound energy to reverberant sound energy in the signal — called the direct-to-reverberant ratio (DRR). Directional cues tell the brain "which way the sound is," but carry almost no information about "how far"; distance judgment relies on a separate basis, and DRR is its primary one.

This is easy to misread as "louder means closer." Loudness is indeed correlated with distance, but it's also tied to the source's own emission strength, making it a vague, weak cue easily confounded by changes in how loud the source itself is being; DRR doesn't depend on emission strength and is a much more stable basis for distance.

Why it happens

In a space with reflective surfaces (walls, floor, ceiling), part of the sound from a source reaches the ear directly (direct sound), and part bounces off reflective surfaces one or more times before arriving (reverberant sound). The farther the source, the faster the direct sound decays with propagation distance following an inverse-square law; the reverberant sound, being the cumulative result of reflections throughout the room, stays relatively stable regardless of the source's specific distance in a roughly diffuse sound field. So as a source moves farther away, the ratio of direct to reverberant energy keeps dropping — this drop forms a usable physical correspondence with distance, which the brain uses to estimate distance.

Directional cues cannot substitute for this function because interaural time difference and interaural level difference are determined mainly by the source's angle relative to the head; at the same direction, whether the source is one meter or ten meters away makes only a negligible theoretical difference to these two cues — they inherently carry too little distance information. If a played sound has all reverberant content stripped out (an anechoic-chamber recording, or simply scaling volume down to simulate "farther"), distance judgment essentially loses its main physical basis, falling back on the inherently vague, weak loudness cue.

Studying it

A common approach, in an environment with controllable reverberation and direct-sound ratio (a real room or convolution-reverb simulation), holds the source's direction fixed while varying only the direct-to-reverberant ratio, has participants report perceived distance, and plots the correspondence curve between DRR and perceived distance; a common control condition is a "volume-scaling only, reverberation ratio unchanged" version, used to test whether loudness alone can support distance judgment.

Common independent variables: direct-to-reverberant ratio, direction (held constant as a control), whether volume scaling is layered on. Common dependent variables: perceived distance rating, the mapping function between actual distance and perceived distance.

Where it stops holding

• The DRR cue's effectiveness depends on the playback content actually containing sufficient reverberant information; for material recorded in an anechoic environment, or with reverberation artificially "dried out" during production, this cue can't operate, and distance judgment noticeably degrades.
• At very close range (near field), the rate of loudness change itself, along with slight spectral tilt at high frequencies from air absorption, also starts to contribute some distance information — DRR isn't the only cue, just the primary one at medium-to-far distances.
• Calibration of the DRR cue depends on a listener's prior experience with a specific room's reverberant characteristics; move to a space with completely unfamiliar reverberant properties, and distance judgment accuracy may drop — this cue should not be assumed to be equally precise in every acoustic environment.

Applying it

• For augmented or virtual reality scenarios that need to convey how far away a virtual sound source is, synthesize or layer in a reverberant tail matched to the target distance (e.g., via convolution reverb processing) — relying on volume adjustment alone to simulate distance will noticeably underperform compared to also changing the direct-to-reverberant ratio.
• When designing spatial audio content for a target scene that is itself anechoic or nearly reflection-free (an open outdoor space, for instance), deliberately add a layer of artificial reverberation or another distance cue to the distance presentation — otherwise users will struggle to perceive changes in distance.
• Verification: have users listen to both a pure volume-scaling version and a version with direct-to-reverberant ratio variation added, and judge which version produces a more noticeable, target-matching sense of distance difference — use subjective distance estimation, not the designer's own listening impression, as the acceptance criterion.

Related

• Same group: A3.14.1 Externalization is the percept that a sound comes from outside the head, not stuck inside the ear canal · A3.14.2 Ordinary stereo headphones struggle to externalize, prone to producing an in-head localization illusion · A3.14.3 The head-related transfer function is individual, and generic parameters reduce localization accuracy · A3.14.5 Binaural recording reconstructs natural spatial cues better than plain stereo
• Nearby: A3.05 Sound source localization
• Search terms: direct-to-reverberant ratio · distance perception · reverberation cue · convolution reverb