Haas effect is a bin-aural psycho acoustic effect regarding the sound source localization. It is known as the “precedence effect” or the “law of the first wavefront” and it was define and described by Helmut Haas in his Ph.D. Thesis in 1949.
Shortly put, our hearing mechanism determine the position of a sound source based on which ear perceives it first and also its delayed “appearance”. As another approach on this, if we listen a unique sound material but one part of the system (left or right) is sufficiently delayed from the other one, we will hear the sound source coming from the part of the head where the earliest sound arrived.
How far on the left or on the right it depends on how much is the difference in ms. This “effect” is working if the sound feed (the unique sound feed) has the same volume in left and the right part of the system and from this come the major benefit of using Haas Effect instead of panning in mixing part of things to add depth, reduce masking and improve the stereo imaging.
Helmut Haas about the Haas Effect / Precedence Effect
In his doctorate dissertation “The Influence of a Single Echo on the Audibility of Speech”, Helmut Hass is talking about the precedence effect / the law of the first wave front:
We can read that the direct sound arriving first at the ear is the only direction determining. This creates only a listening momentum event . The reflection arriving with a propagation delay of more than 2 ms increases the volume in the auditory event also changes the timbre and increases the impression of much bigger spatial extent. Even if the subsequent signal (reflection) has a higher level and arrives with a propagation delay within less than 35 ms, only the signal arriving first determines the perceived incident direction.
Even if Helmut Haas used two speakers for showing this, which were set up at a hearing angle of ± 45° from the listener, his work does not relate to today’s stereophonic method . The localization of phantom sound sources on the loudspeaker base by time differences between 0 and 3 milliseconds in the term stereophony or the equivalence stereophony has nothing to do with this Haas Effect.
The Haas effect is also associated with or sometimes even equated with other terms, “law of the first wave front” or ” precedence effect “.
Law of the First Wave Front
Between two similar audio signals coming from different directions to a listener (for example, a direct sound and its wall reflection), the perceived location of the sound source is assigned to the direction of incidence from which the first wavefront arrives.
This fact is called simply the Law of the First Wavefront or Precedence effect. However, the delay of the second signal must not exceed a certain threshold (echo threshold), since in such a case the listener perceives two individual signals (eg direct sound and echo).
The echo threshold depends on the delay time and the level of the second signal.
The Precedence Effect / Law of the First Wavefront shows that the hearing instrument takes greater account of the direct sound that first coming to the ear when determining the direction of an auditory event (auditory object) than the artifacts arriving later in time. Everything that is arriving a few milliseconds after the direct sound are not perceived as individual auditory events.
Recent studies have shown that habituation to a reflection-prone environment enhances the precedence effect (build-up of the precedence effect), while certain changes in the auditory scenario reset the precedence effect (collapse of the precedence effect).
When a listener gets used to an auditory environment, artifacts that do not provide relevant information to the listener are suppressed. New “unexpected” artifacts are less suppressed because they also contain new information about the environment. An alternative explanation suggests that the build-up of the precedence effect takes place selectively for the directions from which discards arrive at the listener.
The spatial sensation of the localization of sound sources depends essentially on the time of arrival of the sound signals. If the delays are longer than 50 ms, two separate sound events become audible.
Haas’s research does not fully describe the precedent effect. They focus on the conditions under which a delayed sound, which has a higher level than the instantaneous sound, is still perceived from the direction of the instantaneous sound. Only under this boundary condition, if the reflection is “louder” than the direct sound, the specified Haas delay time range of 10 to 30 ms applies.