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The Haas effect

Posted by Duke (D) on September 22, 2001 at 21:10:31

In Reply to: If designed well, can wide speakers image just as well as narrow ones? posted by mesh on September 22, 2001 at 18:42:06:

Hi Mesh,

Your question is a good one, and I agree with your observation that in general narrow speakers do out-image wide ones. Let me try to explain what I think is going on. First a little background...

One of the most fascinating properties of human hearing is a time-gate effect that uses the first .62 milliseconds of an impulse to determine the direction of the sound source, and then largely ignores directional cues from anything arriving after that first .62 milliseconds. This phenomenon is called the "Haas effect", after Helmut Haas, the man who first described it. This .62 millisecond time interval corresponds to the maximum time it would take a sound impulse to travel the roughly 8.5" around the head from one ear to the other. Repetitions of the original signal arriving after that first .62 milliseconds (i.e. reflections) contribute primarily to the perceived timbre, although distinct lateral reflections (off a bare side wall, for example) can cause image shift.

As an aside, let me give you an example of the Haas effect in action: You and I are at opposite ends of a fairly large, reverberant room - in a museum, for instance. You close your eyes as I speak to you. Now, because of the distance between us and the reverberant nature of the room, probably over 95% of the sound that reaches your ears from my mouth is reflected sound. Yet because of the Haas effect, you ears ignore directional cues from those reflections and even with closed eyes you can instantly and unerringly point your finger right at me.

Okay, back to speakers. My point is, that first .62 milliseconds (corresponding to 8.5") is critical to imaging. Any reflections off screws, nearby drivers, or the edge of the cabinet serve to smear the image. Cabinet edge diffraction is usually the worst offender. The most severe smearing of the image would come from reflections or diffractions from discontinuities close to that 8.5" distance (or .62 millisecond cut-off point), while a discontinuity closer in wouldn't smear the image as much. So, either a very narrow speaker, or a speaker with a very wide baffle (like two feet across or so) would theoretically image the best.

It is possible to design a curved or bevelled-edge baffle that largely eliminates image-smearing diffraction. The best example of this approach that I know of is the old Snell Type A, one of the finest speakers of its day, and the first speaker to really show me how much fun first-class imaging can be.

Very few speakers are wide enough to escape having a cabinet edge within 8.5" of a driver. More than likely, a fairly wide speaker that images exceptionaly well either uses bevelled cabinet edges, or has sound absorptive material on the baffle, or uses drivers whose radiation patterns are directional enough to pretty much miss the cabinet edges.

Of course there's much more than baffle geometry involved in designing a speaker that can image well - phase response, alignment of driver acoustic centers, driver behavior (particularly in breakup modes), grille frame, enclosure resonances, mechanical rigidity and even tonal balance all play a significant role.

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