Editing Precedence effect (section)

== History ==
Joseph Henry published "On The Limit of Perceptibility of a Direct and Reflected Sound" in 1851.<ref>Proceedings American Association Advancement of Science vol v, pp 42, 48, May 6, 1851. Reprinted in "The Scientific Writings of Joseph Henry Vol 1," Smithsonian Institution, 1886, pp 296-296</ref>

The "law of the first wavefront" was described and named in 1948 by [[Lothar Cremer]].<ref>
Cremer, L. (1948): "Die wissenschaftlichen Grundlagen der Raumakustik", Bd. 1. Hirzel-Verlag Stuttgart.</ref>

The "precedence effect" was described and named in 1949 by [[Hans Wallach|Wallach et al.]]<ref>Wallach, H., Newman, E. B., & Rosenzweig, M. R. (1949). "The precedence effect in sound localization,"  The American Journal of Psychology, 62, 315–336.</ref> 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&nbsp;ms for clicks, and up to 40&nbsp;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.<ref>Langmuir, I., Schaefer, V. J., Ferguson, C. V., & Hennelly, E. F. (1944). "A study of binaural perception of the direction of a sound source,"  OSRD Report 4079, PB number 31014, Office of Technical Services, U. S. Department of Commerce.</ref> which suggested that if the second-arriving sound is at least 15&nbsp;dB louder than the first, the precedence effect breaks down.

The "Haas effect" derives from a 1951 paper by Helmut Haas.<ref>Haas, H. (1951). "Uber den Einfluss eines Einfachechos auf die Horsamkeit von Sprache," Acustica, 1, 49–58.</ref>
In 1951 Haas examined how the perception of speech is affected in the presence of a single, coherent sound reflection.<ref>{{cite book |title=Fundamentals of musical acoustics |author=Arthur H. Benade |publisher=Courier Dover Publications |year=1990 |page=204 |isbn=978-0-486-26484-4}}</ref> To create [[Anechoic chamber|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&nbsp;s. The test signal (recorded speech) was emitted from two similar loudspeakers at locations 45° to the left and to the right in 3&nbsp;m distance to the listener.

Haas found that humans  [[Sound localization|localize]] sound sources in the direction of the first arriving sound despite the presence of a single reflection from a different direction, and that in such cases only a single [[auditory event]] is perceived. A reflection arriving later than 1&nbsp;ms after the direct sound increases the perceived level and spaciousness (more precisely the perceived width of the sound source). A single reflection arriving at a delay of between 5 and 30&nbsp;ms can be up to 10&nbsp;dB louder than the direct sound without being perceived as a secondary auditory event (i.e. it does not sound like an 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. If the reflection's higher frequencies are attenuated, [[echo suppression]] continues to occur even if the delay between the sounds is somewhat longer. Increased room reverberation time also expands the time span available for echo suppression.<ref>[http://www.aes.org/e-lib/browse.cfm?elib=2093 Haas, H. "The Influence of a Single Echo on the Audibility of Speech", JAES Volume 20 Issue 2 pp. 146-159; March 1972]</ref>