Haas effect [precedence effect]

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(n.) The psychoacoustic phenomena of correctly identifying the direction of a sound source that is heard in both ears, but which arrives at each ear at a different point in time. Direct sound from any source first enters the ear closest to the source, then the ear farthest away. According to German physicist Helmut Haas in the late 1940s, humans localize a sound source based upon the first arriving sound if the subsequent arrivals are within 25-35 milliseconds. If the interlude between sounds is longer than this, then two distinct sounds are heard. The Haas effect is true even when the second arrival is louder than the first (even by as much as 10 dB). In essence we do not “hear” the delayed sound. This is the hearing example of human sensory inhibition that applies to all our senses. Sensory inhibition describes the phenomena where the response to a first stimulus causes the response to a second stimulus to be inhibited, i.e., sound first entering one ear causes us to “not hear” the delayed sound entering into the other ear (within the 35 milliseconds time window). Sound arriving at both ears simultaneously is heard as coming from straight ahead, or behind, or within the head. The Haas effect describes how full stereophonic reproduction from only two loudspeakers is possible. It is also known as the precedence effect.