Haas effect
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The Haas effect is a psychoacoustic effect, also known as the Precedence Effect or law of the first wave front. A listener hears two identical sounds (i.e. identical soundwaves of the same intensity) from two sources: A and B. The sound created at source A, which is closer to the listener than source B, arrives first. To the listener, this creates the impression that A is the only source of the sound.
This effect is often used in audio matrix decoders in surround sound technologies, such as Dolby Pro Logic.
In PA systems the Haas effect can be used to help that the direction of the original signal (localization) is heard unchanged, when the amplified signal is delayed around 20 milliseconds. The level of that delayed signal may be up to 10 dB higher than the direct signal without disturbing the localization.