The precedence effect, Haas effect and echo threshold
When the same sound reaches the ears twice within a short time, the brain does not hear two sounds. It hears one sound from the direction of the first arrival. This is the precedence effect, also called the Haas effect or the law of the first wavefront. It is what lets us find our way in rooms full of reflections.
What happens at different delays
For two identical sounds from two directions, the perceived result depends on the delay between them:
| Delay | What you hear |
|---|---|
| 0 to about 1 ms | one sound, direction in between (summing localisation) |
| about 1 to 30 ms | one sound, direction of the first arrival; the later sound adds colour and spaciousness |
| 30 to 50 ms and more | the later sound becomes a separate echo |
The limit for the third case, the echo threshold, depends on the sound. Speech reaches it at around 30 to 50 ms. Slowly changing music can tolerate longer delays, while sharp clicks produce echoes at shorter times.
The Haas experiment
Helmut Haas, in the early 1950s, played speech from two loudspeakers with a delay. He found that the later signal could be as much as about 10 dB louder before it stopped being dominated by the first signal in terms of direction. So the first arrival wins even when it is quieter, within a window of about 5 to 30 ms.
The effect is not about loudness being fused, it is about direction. A later sound adds to the loudness and changes the timbre.
Where the effect is used
- Delay loudspeakers in PA systems. A fill loudspeaker is delayed so that the sound of the main system arrives first, and listeners hear the direction of the stage. Many engineers add 10 to 15 ms on top of the travel time, see Speed of sound, delay and time alignment.
- Stereo: a delay of more than about 1 ms in one channel moves the image to the other loudspeaker, see Stereo localisation.
- Reverberation: the first reflections increase loudness and spaciousness without changing the direction.
Caveats
- The effect is fragile for very short sounds, such as clicks, or very similar delayed sounds.
- The listener's position matters. Off the centre line, the closer loudspeaker arrives earlier and dominates.
- Reflections between 1 and 15 ms colour the sound through comb filtering, see Comb filtering.
Haas effect or precedence effect?
Haas's name is attached to the observation. The broader phenomenon, including echo suppression, is covered by the term precedence effect. Many books mix the two terms.
Draft: written by an AI assistant, awaiting technical review.
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