The precedence effect or law of the first wavefront is a binaural psychoacoustic effect. It means: If the same sound signal arrives time delayed at a listener from different directions, only the direction of the first arriving sound signal is perceived. The delayed sound signals are localized from the direction of the first arriving signal (the first wave front).
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The precedence effect appears, if the subsequent wave fronts arrive between 2 ms and about 50 ms later than the first wave front. This range is signal dependent. For speech the precedence effect disappears for delays above 50 ms, but for music the precedence effect can also appear for delays of some 100 ms.[1]
In two-click lead–lag experiments, localization effects include aspects of summing localization, localization dominance, and lag discrimination suppression. The last two are generally considered to be aspects of the precedence effect:[2]
For time delays above 50 ms (for speech) or some 100 ms (for music) the delayed sound is perceived as an echo of the first arriving sound. Both sound directions are localized correctly. The time delay for perceiving echoes depends on the signal characteristics. For signals with impulse characteristics echoes are perceived for delays above 50 ms. For signals with a nearly constant amplitude the echo threshold can be enhanced up to time differences of 1 to 2 seconds.
A special appearance of the precedence effect is the so called Haas effect. Haas showed that the precedence effect appears even if the level of the delayed sound is up to 10 dB higher than the level of the first wave front. In this case the range of delays, where the precedence effect works, is reduced to delays between 10 and 30 ms.
The precedence effect is important for the hearing in enclosed rooms. With the help of this effect it remains possible to determine the direction of a sound source (e.g. the direction of a speaker) even in the presence of wall reflections.
Sound reinforcement systems in big rooms use the Haas effect, in order to enhance the sound level for distant locations. For this purpose the loudspeakers at distant locations emit the same sound signal as the direct sound at the listener's location, but the loudspeaker signals are enhanced by up to 10 dB and are emitted 20 ms after the direct sound arrives at the listener. In this configuration the listener will localize all sound from the direction of the direct sound, but he will benefit from the higher sound level, which has been enhanced by the loudspeakers.
The "law of the first wavefront" was described and named in 1948 by Lothar Cremer.[3]
The "precedence effect" was described and named in 1949 by Wallach et al.[4]
The "Hass effect" derives from a 1951 paper by Helmut Haas.[5]