04.04.2011 03:10 PM
Psychoacoustics--The Precedence Effect

Understanding psychoacoustics, how we perceive sound, can lead to better sound system designs as well as creative sound design.

Let’s look at one psychoacoustic phenomenon—the precedence effect.

Consider a listener in a room with reflecting surfaces. Let’s install a simple sound system with one loudspeaker and amplify someone speaking.

Our listener’s ears will take in the direct sound from the loudspeaker as well as that same sound delayed by being reflected off the surfaces of our room. (How the sound is altered as it’s reflected will depend on room surface characteristics, but let’s say for sake of argument that the reflections are pretty good representations of the original direct signal.)

The key point is what our listener actually perceives. Will it be the direct sound and multitude of echoes? Not necessarily. If the reflections arrive at the listener’s ears within about 5 milliseconds (msec) to 30 msec after the direct sound, the sound perceived by the listener will be just the direct sound. This may sound fuller or louder than the direct sound without reflections, but our listener will not perceive individual echoes. As far as our listener is concerned, all of the perceived sound comes from the direction of the direct sound.

Our ear-brain system integrates early reflections with the first sound arrival, and localizes on that first arrival. Depending on the source, the early reflection zone could be extended out to the first 50 msec, or even longer for some music. Beyond this zone, whatever its range, reflections arriving later will be perceived as distinct echoes.

If our sound system designer makes use of the precedence effect, by correct placement of the location and distance of the loudspeaker compared with the person speaking, our listener would localize on the person talking, rather than the loudspeaker.

And a creative sound designer could play with delays, among other effects, to make a stationary sound source appear to move.

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