Must we call them "PZM microphones"? It seems so; like a.c. current.
Crown International, which makes PZMs, neatly avoids the redundancy by the
following usage, "PZMicrophone"--but how do you pronounce that? By
the same logic, we would have "ACurrent" and "DCurrent." Anyhow,
this is Part Two of a PZM discussion, prompted by my experience hearing and
seeing these new mikes in use at the 1981 Oregon Bach Festival in Eugene, where
I went to hear Bach "live" and stumbled upon PZMs as per my account
last month. I came away convinced that this new type of signal picker-upper
can indeed lead to basic changes in the entire art of miking, especially with
classical music but also in all sorts of sound reinforcement and other speech
uses and even in many areas of pop music.
There are some curious aspects of that strange "pressure zone" where the tiny PZM capsule responds to sound, the space within a few thousandths of an inch of a flat plane or barrier, the so-called primary reflecting surface, a space where acoustic sound waves are uncolored by the primary reflections which usually interact to produce interferences, comb-filter patterns of peaks and cancellations which we usually call coloration, especially in the off-axis response. Not gross distortion, of course. But enough to make vast amounts of trouble, as every engineer knows. Remove that coloration, especially at the sides, the off-axis pickup pattern, and you do indeed have a new mike phenomenon with accompanying pickup characteristics that are sometimes startling. That's PZM. There's a kind of infinity involved here. The closer you get to the primary reflecting surface, the higher the frequency of the interferences between direct and reflected waves. At the PZM's tiny distance, these occur well above the range of human hearing. That's the idea, if I'm right. Push 'em upstairs where they're harmless. Some engineers will be wondering, I expect, whether, because electronic circuitry "hears" much higher than we do, there are residual problems with the supersonic. See technical lit, but I expect the PZM people have long since figured that one out for a workable arrangement.
More intriguing, what if you just keep moving your PZM pickup capsule closer and closer to its flat plane? The interferences get higher and higher in frequency, right? Good! So you move still closer until--whoops, you've bumped right into it. Dear me, can't do that. Even if your interferences have now zoomed to infinity. To be sure, infinity is a nice place to relegate unwanted acoustic phenomena but now your mike won't work. It needs room to move, after all.
Still, wouldn't it be nice if .. Now, this is not my idea, and Alan Yordy of KWAX in Eugene merely dropped an aside on the subject during our long rap about his use of PZMs at the Oregon Bach Festival, but . . . it seems that one of our big audio outfits has been experimenting with exactly this idea. Not so impractical! Just dig a little hole, a pit, and mount your mike capsule right in it, flush with the plane surface.
Voila! The infinity-mounted super PZM. (My title, thanks.) Now, I don't know enough acoustical math to say, but I expect that the Achilles heel of such a fine idea might be that little pit, which could set up (relatively) vast reflections of its own, right where you do not want them. Beware of infinity. It's always tricky dealing with a limitless constant.
Yordy (Operations Director of KWAX) first commented to me, on his own use of two pairs of PZMs mounted in clear Plexiglas rectangles, that for stereo--real-time overall stereo, of course--these mikes should be placed considerably farther apart than normal mikes would be.
For his Bach Festival setup, in a moderately large university concert hall ort a big, open stage for a medium-sized orchestra and a chorus (maybe some 50 to 75 people in all), he found the optimum stereo separation was with some 25 feet between the mikes--which is a lot. This jibes precisely with what I'd heard about PZMs, in particular that they have extreme off-axis uniformity, absolutely uncolored sound within a large "half-omni" hemisphere of pickup. At wide angles off-axis, almost all standard mikes are subject to coloration, variably and annoyingly. In a stereo setup, a pair of standard mikes tends to confuse and muddy up the vital center area, since it is "off mike" for both microphones. (Good reason why so many purists use three stereo mikes spaced out overall, even with some loss of stereo separation.) In theory the PZM is absolutely flat all the way out to the sides of its half sphere, and without a doubt it is indeed just that for a very large part of its wide pickup.
Hence--mira bile dictu--you can use a wide stereo separation with only two PZMs and still achieve a clear, unconfused stereo center. As we used to say (before the Surgeon General), put that in your pipe and smoke it.
In addition, because the PZM cuts way down on the confusion of distant sound pickup, again thanks to that uncomplicated clarity of off-axis coverage, it has extraordinary "reach "--indeed this is probably the first thing most users will discover. Clear, sharp details, like whispers, show up at amazing distances (sometimes all too clearly!). Musical sounds are sharp and well defined even though they originate much farther away than normal. This allows a more distant pickup in all situations, from the overall to solo miking, and goes just fine with that wider stereo separation.
I should note that these PZM effects take us a long way towards approaching that binaural clarity of distance reception which our human ears provide via two discrete "audio" channels, an effect that has never been directly achieved by loudspeakers. Just how PZMs compare with actual human hearing is a subject that'll keep us busy for years.
The biggest factor in PZM use is that flat "baffle" and its size. The larger the area of the flat plane, the lower the bass pickup. Alan Yordy's ideal Plexiglas rectangle would be four feet square (and of quarter-inch stock for rigidity) though his actually are somewhat smaller, but the important point is that one can vary the area according to required use. Smaller areas are fine for voices, either chorus or solo, and even smaller for the speaking voice. People don't talk at 16 Hertz. And there are countless other ways to achieve the required baffle "acreage," including the lid of a piano, a flat conference desk or speaker's stand, a wall, floor or maybe even a ceiling mount, just so your half-omni flat response points in the general direction you need. No off-axis coloration, remember.
Well, hardly any. Yordy did remark that in his practical experience the PZM does color the sound a bit at the extreme edge of the pickup hemisphere--but, mind you, that is out sidewise at almost 90 degrees! Compare this with the usual polar patterns for standard mikes.
A few problems do arise with such unwieldy instruments as the Plexiglas--mounted PZMs, even if they are invisible. A big, thick piece of that stuff is not light in weight, and I shudder to think what might happen if one came loose from above and fell on an orchestra, like a French guillotine, perhaps slicing a cello in half along with its player. Yordy's hanging PZMs were, in fact, mounted over the orchestra at the Bach Festival, held in place by a thin guy wire between the two and, presumably, fall-proof.
I assume this placement was to pick up the chorus, in the rear, while the cardioid AKG 414 coincident stereo pair out front picked up the orchestra, and rejected audience noise. Chorus mikes are usually necessary, of course, because the distance ratio between singers in back and orchestra in front is false from close mike range--the live audience hears the music from much further away. But why the PZMs for the chorus, not the out-front overall? Well, I can tell you. Choral sound is one of the "peakiest" of composite musical waveforms with vast quantities of relatively violent acoustic transient intermodulation, coming from all those different and only partially blending voices.
Especially in loud music, this adds up to a mass of what the mike--with the entire circuitry--sees as high-level distortion; worse, the VU meters are treacherous in such situations, reading too low, as I personally discovered to my cost years ago. Serious overloads thus can occur anywhere from mike through to tape or transmitter. Chorus recording is tricky.
The PZM, as noted last month, isn't bothered by high levels; it can take 1 50 dB, and so can its accompanying circuitry. More importantly, there is once again that fabulous non-interference clarity, uncolored in 'every direction over a full half-sphere. Oft-axis, most mikes add coloration, hence both real distortion and a higher level of "peakery" to choral sound, which means exaggerated brilliance, harshness and overload. It often happens. So Yordy was wise to use his PZMs specifically for voice, both for the strong chorus and the solo vocalists.
The standard mikes will do fine for the orchestra.
For the Bach soloists, from one to three or four in a row at the very edge of the stage on one side, Yordy used his second pair of PZMs as described last month. These were mounted on mike stands, desk-like; the Plexiglas, angled upward, was at the singers' ankle level, the stands down on the hall floor a number of feet below the stage. Due to acoustic mixing problems, the "3-to-1" ratio between overall and solo mikes on the same channel, these PZMs were not in stereo but were tied together in mono and fed into both channels, putting the solos in the broadcast center as I heard them. No change in quality of sound when off-axis, no change from one mike to the other three or four feet away as the sound was balanced according to the number or position of the soloists.
Distance was, I'd say, six to eight feet that's a lot. More flexibility, since it minimized the inevitable movements of the performers.
Result: Steady, ultra-clear solo sound, unvarying in position or in level when heard in broadcast, in spite of all the shifting around from one piece to another on the actual stage.
A final word on KWAX's biggest PZM effort, Mendelssohn's vast oratorio "Elijah" (a Bach-related work) which closed the Bach Festival. For this big show there was a larger orchestra, an augmented chorus behind it, and a battery of solo voices--was it six?--lined up in front of the two PZM floor mikes. The Beall Hall stage was jammed to the gills.
But the mike setup remained basically the same. There were two performances on successive nights: I went to the first performance and then listened the next night to the broadcast, all 2 1/2 hours of it.
The results on the air were generally the same as with the Bach, if on a larger scale. Clean, big choral sound from the hanging PZM pair, rock-steady clear solo voices--really remarkable sound even though the other two PZMs now had to cover six singers in a line. But suddenly I noticed a curious effect. On the left stereo side of the orchestra, some of the second violins sounded extremely close and individually sharp, as though from a solo mike only a few feet away. Odd! Because there was no solo mike anywhere near them. As I knew from the previous evening.
Here's what I think happened. Just my guess. Because of the extra crowding, some of the second violins were pushed back, just barely underneath the hemisphere pickup of those overhanging PZMs aimed towards the rear of the stage. Remember the PZM "reach"? And remember Yordy's comment on extreme-edge PZM coloration? That would do it! Am I right, Alan Yordy?
by Edward Tatnall Canby (adapted from Audio magazine, Nov. 1981)
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