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 "precedence effect" was described and named in 1949 by Wallach et al.
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.
Haas found that humans localize sound sources in the direction of the first arriving sound despite the presence of a single reflection from a different direction. A single auditory event is perceived. A reflection arriving later than 1 ms after the direct sound increases the perceived level and spaciousness (more precisely the perceived width of the sound source). A single reflection arriving within 5 to 30 ms can be up to 10 dB louder than the direct sound without being perceived as a secondary auditory event (echo). This time span varies with the reflection level. If the direct sound is coming from the same direction the listener is facing, the reflection's direction has no significant effect on the results. A reflection with attenuated higher frequencies expands the time span that echo suppression is active.[clarification needed] Increased room reverberation time also expands the time span of echo suppression.
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.
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:
- Summing localization: for time delays below 2 ms, listeners only perceive one sound; its direction is between the locations of the lead and lag sounds. An application for summing localization is the intensity stereophony, where two loudspeakers emit the same signal with different levels, resulting in the localized sound direction between both loudspeakers. The localized direction depends on the level difference between the loudspeakers.
- Localization dominance: for delays between 2 and 5 ms, listeners also perceive one sound; its location is determined by the location of the leading sound.
- Lag discrimination suppression: for short time delays, listeners are less capable of discriminating the location of the lagging sound.
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 
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.
Multichannel Audio Decoding 
The effect was taken into account and exploited in the psychoacoustics of the Fosgate Tate 101A SQ decoder, developed by Jim Fosgate in consultation with Peter Scheiber and Martin Willcocks, to produce much better spatiality and directionality in matrix decoding of 4-2-4 (SQ quadraphonic) audio.
Haas kicker 
Many older LEDE ("live end, dead end") control room designs featured so-called "Haas kickers" - reflective panels placed at the rear to create specular reflections which were thought to provide a wider stereo listening area or raise intelligibility. However, what is beneficial for one type of sound is detrimental to others, so Haas kickers just like compression ceilings are no longer commonly found in control rooms.
See also 
- Cremer, L. (1948): "Die wissenschaftlichen Grundlagen der Raumakustik", Bd. 1. Hirzel-Verlag Stuttgart.
- Wallach, H., Newman, E. B., & Rosenzweig, M. R. (1949). "The precedence effect in sound localization," The American Journal of Psychology, 62, 315–336.
- Haas, H. (1951). "Uber den Einﬂuss eines Einfachechos auf die Horsamkeit von Sprache," Acustica, 1, 49–58.
- Arthur H. Benade (1990). Fundamentals of musical acoustics. Courier Dover Publications. p. 204. ISBN 978-0-486-26484-4.
- Haas, H. "The Influence of a Single Echo on the Audibility of Speech", JAES Volume 20 Issue 2 pp. 146-159; March 1972
- Blauert, J.: Spatial hearing - the psychophysics of human sound localization; MIT Press; Cambridge, Massachusetts (1983), chapter 3.1
- Litovsky, R.Y., Colburn, H.S., Yost, W.A., and Guzman, S.J. (1999). "The precedence effect". The Journal of the Acoustical Society of America 106: 1633–16.
- Davis, Patronis "Sound System Engineering", Focal Press; 3 edition (September 20, 2006)
- Philip Newell "Recording Studio Design", Focal Press; 2 edition (December 22, 2007)
Further reading 
- Floyd Toole "Sound Reproduction", Focal Press (July 25, 2008), Chapter 6
- Blauert "Spatial Hearing - Revised Edition: The Psychophysics of Human Sound Localization", The MIT Press; Rev Sub edition (October 2, 1996)
- Litovsky et al. (1999), "The precedence effect" J. Acoustic. Soc. Am., Vol. 106, No. 4