From: DPierce@world.std.com
Subject: re: Stacked Quads
Originator: tjk@csd4.csd.uwm.edu
Organization: University of Wisconsin - Milwaukee
Date: Wed, 22 Jul 1992 13:06:50 GMT


John Stimson writes:
>(Guenter A. Scholz) writes:
>>  Re. the mid/tweeter cavity, I would think (I have 
>>not measured this) that diffraction is not important for an extended source
>>it simply beams too much.  Diffraction is a very important problem mostl
>>for point sources.
>
>I guess you haven't seen what a diffraction grating can to to a column of
>light then.  If the Quads are like most (all) electrostatics I've seen,
>the drivers are shaped like a big grid, which would make a wonderful
>diffraction grating!  Not sure of what frequencies this would affect the
>most, since I don't know the spacing of the mesh...
>

Well, it might be significant, but not considering the actual size and
spacing of the mesh. The Quads use a perforated grid, the holes are
approximately 2 mm in diameter and spaced about 4-5 mm apart on a triangular
grid. Given those dimensions, one would not expect and significant 
"diffraction" effects until well above 20 kHz. The interference effects
generated by such things as the cabinet frame, the recessed tweeter panel,
and so forth happen down in the region of 4-5K and above, so ar far more
significant.

On another note, while I appreciate Guenter Scholzs' (what seems to be)
genuine concern about the modification issues I discussed here, I think
his assertions are incorrect. For example, he made at least one testable
assertion regarding the removal of the damping in the rear of the Quads:
That they are ESSENTIAL for dampening the low frequencies and removing them
can only result in loose and (to use his term) "one note bass."

Okay, this is indeed a testable hypothesis. If he is correct, then we ought
to be able to measure a significant and quite obvious peak in the response of
the system. While I do not have the curves around any more, I did measure
several of the systems I performed this modification to and not a single
one of them exhibited any evidence of the phenomenon Mr. Scholtz predicts must
happen. Further, it was interesting to note that the response above 50 Hz
was smoother and showed far fewer perturbations, most especially in the Aregion
between 150 and 500 Hz, where the padding seemed to be quite reflective.

In short, when Mr. Scholz's hypothesis is tested, it fails. I suggest that
the basis of the hypothesis similarily fails.

Further, there is an implication that the dampening might somehow be safety
or reliability related. Of the 3 systems that I modified and that I have been
able to keep tabs on, every single one of them are still working (as of about
4 years ago, one owner died, his widow sold the setup and I have no idea where
it went). The most that they have required was an occasional tightening of
the dust wraps on the bass panels, something which unmodified Quads required
with equal frequency.

Let me assure Mr. Scholtz that the modifications I performed were no casually
contrived, hastily performed "experiments." I had at the time more than
sufficient instrumentation, experience and knowledge to understand clearly the
issues involved and the means to verify the results of the actions taken.

Let me sya this in agreement with Mr. Scholtz: Modifying or contructing
electrostatic loudspeakers is not in the same league as working on other
types of loudspeakers: there is an added factor of simple and obvious
life-threatening danger involved. Sitting in the back of the Quads is
up to 6,000 volts worth of potentially lethal electricity waiting to bite
you. The current availability is low, so the chances of a fatal accident are
cimilarily low, but the potential for a tragic accident is there. I do not
recommend nor condone the indiscriminate modification of Quads or any
speaker of that type. Leave this to experts or pick something a little more
benign to play with.


|                Dick Pierce                |
|    Loudspeaker and Software Consulting    |
| 17 Sartelle Street   Pepperell, MA  01463 |
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