Delays shorter than ~25-35 ms are heard as timbre or doubling, not as distinct echoes

The Haas effect defines a psychoacoustic threshold: when a repetition arrives within roughly 25-35 ms of the original, the listener fuses both into a single perceptual event — the delay appears as a change in timbre or apparent width rather than a separate echo. Above ~35 ms the repetition becomes a distinct sound. This threshold governs the difference between flanging/chorus (1-30 ms), slapback (10-120 ms), and perceptible echo (over 100 ms). The exact threshold depends on the sound’s attack: slow-attack sounds require longer delays before the repetition is clearly perceived.

Examples

A 15 ms delay on a drum hit creates a flanging or room-width effect. A 50 ms delay on a flute may only be perceived as echo above ~80 ms due to the slow attack. A 400 ms delay on dry guitar yields a classic rockabilly slapback echo.

Assessment

A producer sets a delay to 20 ms. Predict what a listener perceives and explain the psychoacoustic reason. At what minimum delay would a separate echo become clearly audible on a slow-attack pad?