Precedence effect

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.

The "law of the first wavefront" was described and named in 1948 by Lothar Cremer.

The "precedence effect" was described and named in 1949 by Wallach et al. They showed that when two identical sounds are presented in close succession they will be heard as a single fused sound. In their experiments, fusion occurred when the lag between the two sounds was in the range 1 to 5 ms for clicks, and up to 40 ms for more complex sounds such as speech or piano music. When the lag was longer, the second sound was heard as an echo.

Additionally, Wallach et al. demonstrated that when successive sounds coming from sources at different locations were heard as fused, the apparent location of the perceived sound was dominated by the location of the sound that reached the ears first (i.e. the first-arriving wavefront). The second-arriving sound had only a very small (albeit measurable) effect on the perceived location of the fused sound. They designated this phenomenon as the precedence effect, and noted that it explains why sound localization is possible in the typical situation where sounds reverberate from walls, furniture and the like, thus providing multiple, successive stimuli. They also noted that the precedence effect is an important factor in the perception of stereophonic sound.

Wallach et al. did not systematically vary the intensities of the two sounds, although they cited research by Langmuir et al. which suggested that if the second-arriving sound is at least 15 dB louder than the first, the precedence effect breaks down.

The "Haas effect" derives from a 1951 paper by Helmut Haas. In 1951 Haas examined how the perception of speech is affected in the presence of a single, coherent sound reflection. To create anechoic conditions, the experiment was carried out on the rooftop of a freestanding building. Another test was carried out in a room with a reverberation time of 1.6 ms. The test signal (recorded speech) was emitted from two similar loudspeakers at locations 45° to the left and to the right in 3 m distance to the listener.

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