No Fragments Archive 10: Diskmags/nf_archive_10.iso/MAGS/KELSTAR/KELSTR01.MSA/TEXT/RSPZM.TXT

Text File | 2008-03-13 | 19KB | 387 lines

ERRR,this is another downloaded file,this time regarding Pressure
Zone Microphones,(I only know what PZM stands for `cos I read the
first few paragraphs before my brain melted).I sicerely hope that
there is somebody out there who knows what they are,what they do,
and therefore might enjoy this article.*Bob*





PZMs -- Definitely Not Your Daddy's Oldsmobile
==============================================

Since there seems to be a healthy curiosity on the Forum
concerning Realistic (Radio Shack) PZMs, and PZMs in general,
this discussion is my input to the topic based on about 12 years
of experience, experimentation, research, and field (literally)
use.

I'll try not to insult anyone's intelligence. I know some of you
are old-timers in the sound business [(I've been doing this stuff
for about 22 years. Design, installation, consultation, live &
studio recording, live SR for theater (both large scale and
small), -- you name it. No desire to impress here -- just a
little background.)] But I also know that many may have just come
into this business, are curious, and have a desire to learn. So
to those that may already know some of what I discuss, my
apologies. (I can hear it now --- "Yeah, yeah, yeah -- so get on
with it!")

I believe child-like curiosity is the most vital natural trait
one can have in this or any business. (Well . . . a sense of
hearing helps.) With that, let me raise the flag and state that
"We need good people in this business." We need people who want
to learn, are willing to take chances, make mistakes, and further
both the art and science of audio. To those people on CIS that
fit that description, I dedicate this discussion.

=============================

The original question that inspired my response had to do
specifically with Radio Shack PZMs. Before getting into the
details of those, perhaps a little background in PZM "theory"
would help those whose education on the topic is incomplete.
Anyone who wants to cut to the chase, go directly to the heading
"Radio Shack PZMs" later in this document.

PZM History
===========


For the acronymally-disinclined (a new politically-correct
term?), PZM means "Pressure Zone Microphone". It really isn't a
trade-mark (although I think Crown now holds the rights to it
when used in a product-name context). It's a theoretical type of
microphone pickup technique.

Without getting into a lot of acoustical theory, the basic
principle relies on the pressure wave produced by all sound
sources being confined by a "boundary layer" that directs the air
pressure into a transducer element (mic element) at very close
proximity to the boundary layer. It takes advantage of sound's
wave property that we've all seen in our 7th-grade science book.
The wave spreads out in all directions equally from a single
point, omnidirectional source. Remember those drawings of waves
in a pond? Since the element's active surface plane (diaphragm)
is mounted very close (about the thickness of a business card),
to the reflective boundary, a large proportion of the wave is
directed into the element, without a great deal of degradation of
frequency information. That's the theory in a nutshell. Actual
practice is pretty close to the theory.

A fellow named Ken Wahrenbrock was experimenting in the 70's with
this idea. He was really the pioneer in the practical aspects,
and was trying to get the sound industry interested in applying
the idea. As we all know, a radical departure from the status quo
is usually met with skepticism and ridicule. Altec-Lansing sent
out the elements as tie-tacks to its dealers. I remember getting
some of them.

But a few curious folks took Ken seriously, and experimented with
several configurations. Some issues of DB magazine (if I remember
correctly --  maybe it was RE-P) covered some systems that used
PZM elements mounted on various plexiglass sheets and flown over
orchestral ensembles, etc. Getting favorable press in industry
publications provided a great boost to the idea. I think there
were also some AES papers written.

As it turned out, Crown wound up with the rights to the design.
Again, if memory serves me correctly, Wahrenbrock sold it to
Crown. Some details are probably missing here -- some of you know
the details, I'm sure. Be that as it may, most PZM
implementations you see today carry Crown's blessing (if not
their logo), although not all are built by Crown.

For those who are interested, Crown published quite a nice series
of technical articles on PZMs and their use. Write to or call
Crown and get them. Credit goes to Bruce Bartlett. Of course, the
slant was to their own products, but the application notes apply
to most, if not all, PZM devices. And, by the way, I am a
longtime Crown user. I have much respect for the company, their
products, and their founding philosophy. For those new to the
business, a good history trivia research project would be to find
out why their name is what it is. It's not politically correct,
but gives an insight into their mission. (that last word is a
clue . . .)

Moving right along . . .

My (Early) Experience With PZMs
=======================

For the last 15 years, I have been audio contractor/engineer for
a 6,000 seat outdoor amphitheater here in southern California.
The facility was built in the late '20's. Programming ranges from
classical recitals to symphonies to Broadway musicals and opera.
After an unfortunate vandalism incident in 1981, we had the
opportunity to upgrade the technical capabilities. Up until that
time, stage pickup for the musicals and operas was limited to
"mousing" regular mics at the downstage edge of the performing
area. Since it is impractical (and virtually impossible) to fly
anything above this stage, this was a reasonable approach. It
worked moderately well, but required considerable effort at the
board.

During the overhaul, I suggested the PZM approach. We purchased 3
Crown elements mounted in angled plexiglass enclosures. The
performance of these has been incredible. I never fail to receive
compliments on their quality. And to this day, they have not
failed.

Since 3 units was not enough to cover some of the more ambitious
shows attempted after the first year, we had to make a decision.
Buy more or rent/buy wireless. This facility is operated by a
non-profit community music association. The money didn't exist to
purchase more Crowns at $400.00 each.

Since I happen to drop in to Radio Shack stores on an irregular
basis, I noticed they were offering a PZM at $39.00. Willing to
take a risk at 1/10th the cost of the Crowns, I figured I
couldn't go wrong. I think I was the first person (at least in
the Riverside area) to show an interest in them. The sales person
didn't even know what they were. When I asked to see the box and
the paperwork, there was no hesitation. See, from experience with
earlier RS stuff, I knew they were pretty good at putting some
kind of circuit diagram in certain things. Well, I was right. The
operator guide had a circuit diagram with the information I
needed.

I was looking to see if the output was balanced. See, they come
out of the box with a single-circuit 1/4" plug. I can only assume
they were marketing these to home/hobbyist types. The diagram
settled the issue for me -- the output was shown as transformer
balanced, 600-ohm. I bought four of them on the spot. I would
have bought 6, but four was all they had at that store.


The Chase -- Radio Shack PZMs (Including My (Later) Experience
With PZMs)
==========

I immediately re-wired them with Switchcraft A3Ms. I just cut off
the 1/4" about 8" from the end (saved the molded connectors and
pigtail for making other adapters), wired shield to pin 1, 1/4"
tip to pin 2 and 1/4" sleeve to pin 3 - standard configuration.
Put in specified AA battery, plug into the board, and Voila! I
was immediately impressed.

Continuing my experiment, I un-mounted the elements from the
plates -- just two little screws. I waved them around in the air,
and all over the place, and just listened through headphones. I
put them up against walls, windows, and any other flat or rounded
surface I could find. I taped them to the side of the kitchen
sink, to the bottom of the (sorry . . .) toilet lid and just
listened. Well . . . running water contains a lot of high-
frequency energy. (Hey, it was valid . . . ) I even taped them to
the underside of the hood of my car and ran down the freeway
recording the sound of the engine. (You can't even believe the
noise even a quiet rice-burner makes under the hood!) When I had
satisfied myself that I could make these work, I started working
on different mounts for serious applications.

Since I do a lot of location recording with a minimalist setup, I
thought an interesting experiment would be an omnidirectional
stereo mic array. So, I put two of the original plates back to
back, drilled a hole to accept a standard EV mic mount (sans the
clip part). The thickness of the two plates together is just
about the same as the gap in the mic mount. A thin washer on each
side as a shim, and I had my mount.

I tried it on a choral rehearsal for which performance I had
contracted to record. I took a portable machine, and just set up
the array in about the right place, and recorded. Upon listening
to the tape, I was again impressed. Although not super strong in
the bass range, the imaging and presence was stunning. The
lacking bass is attributable to the plates being only 4 or so
inches square. PZMs need a pretty large boundary layer to collect
the bass. Remember, low frequencies have a pretty long wave-
length. Four inches square isn't very efficient for the low-end.

I still use this configuration in certain circumstances. And it
still performs very well, especially considering the price.

Now, back to the amphitheater situation.

I measured the plexiglass shields used with the Crowns. I bought
the necessary acrylic sheets, tools, and adhesive. I proceeded to
build my own enclosures. I mounted the RS elements at the right
spot. My next opportunity to use them was our next summer season
at the Bowl (in Redlands).

I set them up with the Crowns for one of the dress rehearsals. As
I already had plenty of experience with the EQ and gain settings
for the Crowns, it wasn't much of a task to get them in the
ballpark. A little tweaking, and away we went.

Here's what I found:

1) The RS required a little lower gain setting. The Crowns are
phantom-powered through the board, 24VDC in this case with a
Tangent 2402. Had the Crowns been powered with the specified 48
VDC they would probably have a little higher output.

2) The RS needed a little more high end boost -- not much,
though.

3) After adjustment, there was no practical, noticeable
difference.

Over the years since that first experience (1983), I have used up
to 5 (including the original 3 Crowns) across a main stage area
60' wide and 40' deep. At times, I have used 7 across the entire
playing area (about 85'), with 3 or four (in their original
configuration -- attached to the plate) taped to set pieces and
painted to match the set.

I am continually re-configuring mine (I have eight now),
depending on the project at hand. This past summer, we did "La
Boheme" presented in "concert" form. The orchestra was centered
upstage of the players. There were no flats -- just a few
props/set pieces. I mic'd the whole thing with the 3 Crowns. The
RS would have been just as good, but I would have to had taken
them apart from their then present configurations. Too much work.
I had planned to set up a couple of SM 81 overheads on the
orchestra, with a couple 451s on strings and winds. But when I
actually saw the layout and blocking, I couldn't foul up the
sight lines with mic stands. I suggested to the director that we
could use the PZMs but I couldn't guarantee the balance between
singers and orchestra, since the mics would pick up whatever they
heard. As it turned out, it was the most successful program of
its type I've ever done. It was so successful, that on the
symphony program the next week, I lost my head completely, and
used the same setup but added a pair of AKG 451s on the winds.
The audience (of about 4500) loved it. I got nothing but
compliments. So, even after years of experience, I'm still
learning ways to use these things.

Again, the RS PZMs would have substituted just fine.

Here are a couple of the other configurations I've come up with,
and that work very well, all things considered.

1) A pair of elements mounted on a plexiglass surface about 2-
1/2' X 2'. Space the elements about 18" apart across the panel
the wide way. Use a mic clip mount (no clip) as above. I've used
this very successfully mic'ing jazz/big band drums, in
combination with a pair of overheads. With the right placement,
you get a good stereo image (but not too wide) and a good balance
between percussion elements (assuming a player under control).

I've also used this to mic soloists in concert situations. Again,
a good stereo image, a single stand, and quasi-invisibility
(depending on stage lighting).

2) Elements mounted as necessary on stage pieces. I've already
mentioned this. They work very well on table tops, on door jambs,
or just about anyplace where you have to pick up non-moving
dialogue on stage. I did "The Music Man" a couple of years ago
(just once of many), and used this technique with great success
on the Paroo house set.
3) Elements mounted in/on just about any surface. Piano lids or
the floor under a piano works well. A couple of caveats, though.
If you try to gaffer tape a PZM to the lid of a $100,000 Steinway
or Bosendoerfer, be ready to find another line of work. On pianos
that you can do this to, full stick is good to start. You can
work on finding the sweet spot as you go. If you stretch this
out, just think how much money you can make, all the while
insisting that "since all pianos have their own characteristic
sound, you need to spend this time making sure you are getting
the best possible sound. You DO want the BEST sound -- DON'T
you?" On the other hand, if they look at the clock and then ask
what you're doing, just say, "It's technical."

Seriously, though, experiment with PZMs on piano. There is really
no right or wrong way. Lid up, lid down. Soundboard, floor, wall
adjacent. Whatever gets you the sound you want. Just use common
sense and your ears. Don't be afraid to experiment. Part of the
neat thing about PZMs is their pickup field. Regular mics tend
very much to have directional characteristics -- even omnis.
Pianos are a big instrument. The whole thing is really the sound
producer. Depending on your goals, PZMs can solve some otherwise
thorny directional problems with pianos. As Professor Hill
admonished, "Think men -- just think." (no sexism intended . . .)


Technicalities
==============

Far be it from me to suggest that a PZM at any price will do
anything, anywhere. They are tools. Use them right, and you're a
craftsman. Use 'em wrong, and you're a hack. (And sometimes
unemployed.) Don't be afraid to read and heed the suggestions
printed in the material that comes in the package. A lot of it
appears to be vague and general. Try reading the operator manual
that comes with a U-87. Much of audio is vague and general. Mic
selection and use is as vague and general as it gets. The real
point is to use what you have to get what you want.

With the RS PZMs or any other mic, try it in as many situations
as possible. You'll figure out which are lousy soon enough. At
$40 or $50 each, just about anybody can afford one. And if you
don't like it, make a tie-tack out of it. Most craft shops have
glue that will stick to the plastic.

A question was raised as to SPL capabilities. Although I have
never used anything but the AA battery (new ones at each critical
use. Some of mine have operated on the same battery after two
years in storage), RS sells a battery (#23-469) that raises the
maximum SPL. Install two of the batteries in place of the AA.
It's really the preamp that has the overload problem. The higher
voltage lets it breathe a little. All you op-amp techno-dudes
know what I mean.

They are not phantom-powerable without a complete redesign and
re-construction, to my knowledge. The semiconductors chosen are
basically backwards in polarity. I checked into it with my first
units. There is talk of someone who has done a modification.
Okay, but why spend the time and money on a $40 mic? And if you
screw it up -- geez! (Although, if anybody knows about this, I'd
sure like to know, too.)

I've done a few sketches of some enclosure/mounting ideas that I
have tried. Anybody interested, send me E-mail with an address
and I'll send them to you. Use them, or come up with your own.

These are great little mics for the price (that's the catch). If
you expect them to take the place of $8000 Manleys or even cheap
old $1500 Neumanns, you may be expecting too much. I would never
expect them to ever get into the "world class" category. (For
that matter, are you? Okay, so you're Ross Perot's son . . .) But
if you are willing to experiment, use some imagination, take the
time to learn their limitations and potential, you'll have some
valuable tools in your pocket.

We all know the levels of elitism in this business. Snobbery runs
deep in some circles. If you are uncomfortable with the Radio
Shack logo, paint over it. Or scratch it off (try to make it look
like natural wear -- it's much more convincing that way.) But by
all means, take a chance. It's hard to have a good dinner out for
$50. And after dinner, what? Hey, in certain parts of the
country, for $50 you could bypass dinner altogether and go
straight to . . .  well, you get the point. Buy a PZM -- no,
invest in one, or two or four. Learn what you can do. Later, when
you can afford something different, step up to the counter. In
the meantime, let these help pay for themselves.

Here are the printed specs:

Freq. response: 20 - 18,000 Hz (no 3 dB down points published)

Impedance: 600 ohms

Sensitivity: -74 dB (ref.: 0 dB=1v/mbar, 1k)

Pattern: Hemispherical

Battery life: 2,000 hours

=================================================================

Toivo Maki
Intermedia
PO Box 4535
Riverside, CA 92514

CIS 73467,2641