From: DPierce@world.std.com (Richard d Pierce)
Newsgroups: rec.audio
Subject: Re: Interesting Speaker Question (to me, at least)
Keywords: Interference
Date: 11 Oct 92 21:54:50 GMT
Organization: The World Public Access UNIX, Brookline, MA

In article <5618@ssc-bee.ssc-vax.boeing.com> bruce@ssc-vax.boeing.com (Bruce Stock) writes:
>
>I have been puzzling without success over the situation described below,
>so it is time to expose my ignorance and ask the net for illumination:
>
>Eons ago in freshman physics I remember there was a textbook example of
>wave interference.  It involved a pair of slits which intercepted a
>wavefront, thereby producing an interference pattern at a downstream
>plane, so long as the width of each slit was reasonably less than the
>wavelength.  (I'm sure you all remember this, so I won't include the
>tortured ascii graphics)
>
>It occurred to me that this phenomenon could also appear in the
>soundfield of any stereo speaker system, if certain conditions prevail.
>For example,  when a sustained note appears at frequency where the
>speakers act as a point, or line, source.  Yet, I have seen no
>discussion of this effect in the popular audio literature.  So I assume
>that there must be real world considerations which render the ideal
>situation inaudible.  For example, real instruments do not often produce
>a single frequency, so the effects might not be that obvious.  And of
>course real living rooms have floor, ceiling, backwall, and sidewall
>reflections to contribute (unless near-field listening is used?).

The situation is far more complex than you are assuming here. First, the
speakers (and most instruments) do not act as point sources except at
very low frequencies, where the wavelengths are very large compared
to the radiating source, the speaker, the distance from the source to the
detector, and the detector itself (our ears). Secondly, this detector,
(our eras, again) is in one sense a pretty powerful detector (it provides
sub-millisecond correlation operations from at least two points, for
example), yet is at the same time sufficently stupid to be fooled into
perceiving distance, lateral and vertical placement, etc. with what turns
out to be some easily generated cues.

All of these complications contribute to a remarkably complex sound field
being generated, detected and interpreted.

>On my own speakers (Dahlquist DQ-8's) I do hear something when I put on
>intercarrier hiss (poor man's white noise), and move my head from side
>to side about the midpoint.  It isn't the classic interference pattern
>that I would expect, but more like a weak sin(x)/x amplitude variation.
>
What you are hearing is localized (both in space and in frequency) nulls
caused by multiple-source interference, by actual defects in the output of
the drivers themselves and so on.

>So how about it gentlemen - why is this effect not the kiss of death for
>detailed, yet stable imaging?

Well, it is. The actual imaging information is pretty poor, but were are
willing to allow ourselves to be pretty gullible when it comes to being
handed cues and interpreting them. 

-- 
|                Dick Pierce                |
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Newsgroups: rec.audio