The IMP Arrives
"Oregonian Haruspex" wrote in message
...
What sort of test setup did you use to measure the results of your design?
Do you have a favorite set of test mics? What sort of measurement
instruments do you use to confirm that your speakers match the
mathematical model of how they should perform?
Dear Oregon,
As I believe I have mentioned, I did not build them, a very talented DIY
speaker builder in Indiana built them to my desires. He has not written up a
description of the design factors, just said some minimum facts as follows:
"I haven't written anything on the IMP. I certainly have my crossover
schematics, but that is really not something I would expect to see posted
for something like this. It would make sense to write about how the back
panels share one passive crossover and each front panel has its own passive
XO, with adjustable lpad on the amp side, before the crossover, to allow for
control of the radiation pattern. Perhaps something how the cabinet is
divided vertically into 4 triangular shaped enclosure spaces, one for each
MTM to reduce standing waves. I could include language about the octagonal
shape with large 45º adjacent to all driver panels to reduce diffraction.
The MTM configuration to reduce the effect of ceiling and floor bounce
cancellations. Also, that each panel is designed such that the frequency
response is essentially flat on axis, as is the FR when measured in the
center between any two sets of drivers, when only those two panels are
turned on. It was assumed that power response would be relatively flat in
room with this approach and the long gated frequency response measurements
appear to support this. Anyway, I'm not sure there would be that much to
include in technical details."
I know he uses the windowed MLS technique for measurment, but not which
mikes. This one may have been easier to measure because it is just a
satellite speaker and you don't need to worry about the deep bass response.
But the design goal had not so much to do with frequency response as
radiation pattern. We start with a basic box radiating in four directions
evenly, then put some L-pads on the front two panels to be able to attenuate
the frontal pattern in a way that also gives the distance/intensity trading
that Mark Davis called for in the Soundfield One design. We chose to
attenuate the inner front by 3 dB and the outer front by 9 dB, so that as
you go from center to either side, you get more in the loudness from the
opposite channel and less from the one you are near. This compensates for
how close you are to the one speaker and makes the center imaging remain
stable.
So we took a guess as to how to make this pattern work and we haven't done a
careful radpat measurement yet to see if we are getting it very precisely,
but audibly it works quite well, because we are getting that "out of body"
projection of the sound outside the speakers themselves by means of
reflection of the rear wave, and also even imaging as you go around in the
room due to my required speaker placement and the frontal radpat of the
speakers. This is a combination of the design features of the Bose 901 and
the DBX Soundfield One, and an improvement on both. I also listen in
surround sound for even better spatial results.
Gary Eickmeier
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