Mike Rivers wrote:
In article writes:
It was Eric Benjamin, you may have heard of him, and it was
published measurement data, not measurements he did himself.
Yes, I know Eric Benjamin (the one at Dolby). But I forgot why you
brought this up.
Because he was the one that reported this testing of the PZM
which found it wanting in comparison to a simpler omni. I
mentioned his name because if you know him you know that he
rarely, never in my experience, reports anything that
diverges from fact. I also know him personally and know
that his nature admits no bull****.
No. They didn't have links back then. I'm sure the PZM was developed
experimentally with some analysis to show that the principle works. I
doubt there was any sort of optimization analysis. Someone who worked
with him at the time that he was hand-building PZMs said that he used
a dollar bill as a gage to set the spacing of the capsule off the
boundary plate. Not very scientific, but reasonably repeatable. Or
maybe it was his first dollar.
How about Cain's analysis? You'te pretty good at that
sort of thing.
Here's where my doubt about all this happens. There is no
difference at all between a PZM on a wall and two summed
capsules on cantilevers in free space facing each other with
twice the gap between them and where the room on one side of
the pair is a mirror image of what's on the other side. I
can't see how the PZM configuration offers anything positive
compared to a single omni at that position. The fact that
sound approaching the PZM at any angle other than
perpindicular reaches the opening at the center of the
capsule with a variety of delays only implies combing at the
highest frequencies. The cantilever itself introduces
diffraction and angular variations that reach lower.
Absolutely no benefit from this configuration is apparent to
me in theory.
What screws up the response relative to flush mount in
practice is the diffraction due to the cantilever that holds
things above the gap. The dimensions of that are such that
the effects reach down a ways.
I'm sure that Crown has frequency response and polar plots. Have you
looked there?
I've learned, as I think you have too, not to pay much
attention to manufacturer's plots.
The design was from the days when they didn't have a lot
of graphic artists so what you can find, if you can find old data, is
likely to be close to what you get. Maybe someone with a well indexed
library of audio magazines going back to the '70s can find a good
review of an early generation PZM - maybe a Studio Sound issue. I
might even have one myself.
It really isn't a review I'd like to see but some
theoretical acoustical analysis that defines the difference
and the benefits quantitatively.
The principle of the PZM (as it was explained to me by Farrell
Becker, an associate of Ed Long's at the time, back stage at
Wolf Trap) is that the path length from the source to the capsule
is independent of the distance between the source and the
microphone.
The same is truer with an omni having a small orfice at the
same postion.
Since everything that gets to the capsule (theoretically)
is reflected off the boundary plate, everything travels through the
same path which is much shorter than the shortest wavelength
supported by the microphone. So there is no phase cancellation
at certain frequencies due to the path length. The idea was to
make a microphone with flat frequency response, at least down to
the lowest frequency that could bounce off the boundary plate.
This is the hand waving (not yours, I know) that really
makes no acoustical sense. Reflection from the plate or a
wall on which it is mounted just gives rise to a virtual
source on the other side which is summed with the real
source at the capsule itself. Imagine the wall as a mirror
and you'll see what the mic hears.
I'm sure those worked fine for the person
who wrote the article, and it makes good copy.
That's what I think. It was a cute marketing ploy, an
intruiging looking widget with no substantial real
difference that isn't of a somewhat negative nature.
Bob
--
"Things should be described as simply as possible, but no
simpler."
A. Einstein