The precedence effect or law of the first wavefront is a binaural psychoacoustic effect. When a sound is followed by another sound separated by a sufficiently short time delay (below the listener’s echo threshold), listeners perceive a single fused auditory image; its perceived spatial location is dominated by the location of the first-arriving sound (the first wave front). The lagging sound also affects the perceived location. However, its effect is suppressed by the first-arriving sound.
The Haas effect is a psychoacoustic effect, described in 1949 by Helmut Haas in his Ph.D. thesis. It is often equated with the underlying precedence effect.
Sound reinforcement systems
Haas’ findings can be applied to sound reinforcement systems and public address systems. The signal for loudspeakers placed at distant locations from a stage may be delayed electronically by an amount equal to the time sound takes to travel through the air from the stage to the distant location, plus about 10 to 20 milliseconds and played at a level up to 10 dB louder than sound emanating from the stage. The first arrival of sound from the source on stage determines perceived localization whereas the slightly later sound from delayed loudspeakers simply increases the perceived sound level without negatively affecting localization.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.