Haas Effect

The Haas effect is a psychoacoustic effect related to a group of auditory phenomena known as the Precedence Effect or law of the first wave front. These effects, in conjunction with sensory reaction(s) to other physical differences (such as phase differences) between perceived sounds, are responsible for the ability of listeners with two ears to accurately localize sounds coming from around them.

When two identical sounds (i.e., identical sound waves of the same perceived intensity) originate from two sources at different distances from the listener, the sound created at the closest location is heard (arrives) first. To the listener, this creates the impression that the sound comes from that location alone due to a phenomenon that might be described as "involuntary sensory inhibition" in that one's perception of later arrivals is suppressed.

The Haas effect occurs when arrival times of the sounds differ by up to 30–40 ms. As the arrival time (in respect to the listener) of the two audio sources increasingly differ beyond 40 ms, the sounds will begin to be heard as distinct; in audio-engineering terms the increasing time difference is described as a delay, or in common terms as an echo.

The Haas effect is often used in public address systems to ensure that the perceived location and/or direction of the original signal (localization) remains unchanged. In some instances, usually when serving large areas and/or large numbers of listeners, loudspeakers must be placed at some distance from a stage or other area of sound origination. The signal to these loudspeakers may be electronically or otherwise delayed for a time equal to or slightly greater than the time taken for the original sound to travel to the remote location. This serves to ensure that the sound is perceived as coming from the point of origin rather than from a loudspeaker that may be physically nearer the listener. The level of the delayed signal may be up to 10 dB louder than the original signal at the ears of the listener without disturbing the localization.

The Haas effect also explains why it is possible to simulate a complete complex audio field from only two sound sources in stereophonic and other binaural audio systems. It is also utilized in the generation of more sophisticated audio effects by devices such as matrix decoders in surround sound technologies, such as Dolby Pro Logic.

For a time in the 1970s, audio engineers used the Haas effect to simulate that a sound was coming from a single speaker in a stereo sound system, when it was actually coming from both. This was to compensate for the fact that a sound coming from a single speaker would be 3 dB lower in volume than a sound coming from both. This technique has problems if the stereo sound is mixed to mono, as a comb filter effect would occur. Also, the aesthetics of sound mixing changed to exclude the use of solo instruments emanating from a single corner of the sound field in most popular recordings.

The effect is named after Helmut Haas who described the effect in his doctoral dissertation to the University of Göttingen, Germany. An English translation was published in December, 1949 as The Influence of a Single Echo on the Audibility of Speech.