To an untrained eye, a Pressure Zone Microphone (PZM) looks a bit like a switch plate cover or other piece of hardware. However, these unique and often underappreciated microphones provide a host of creative solutions and opportunities in the studio. This month, we’ll try out some PZMs and find out how they can help broaden your capabilities for recording just about anything that comes your way.
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An early prototype PZM mic developed by Ken Wahrenbrock.
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Before we look at various applications for PZMs, it’s useful to consider what led to their invention. In 1978, two engineers, Ed Long and Ron Wickersham, discussed what became dubbed as “pressure zone recording,” which consisted of a microphone element mounted less than one millimeter away from a boundary, such as a metal plate.
The resultant microphone would combine both the direct and reflected sound waves hitting its capsule and virtually eliminate any variation in the timing of the sounds. This process eliminated any comb filtering, which occurs when the direct and the delayed or reflected sound arrive at a mic’s capsule at slightly different times. That results in a smearing or muddying of the original sound. By their design, PZMs eliminate this time differential resulting in no comb filtering in the audible frequencies.
The results of the first manufactured Pressure Zone Microphone, developed by Ken Wahrenbrock, and appropriately dubbed the PZM, were impressive. In the early production models made by Crown, the mic picked up direct and reflected sound waves in-phase, delivering accurate, realistic response over a frequency range of roughly 50 Hz – 15 kHz.
By combining the direct and reflected sounds, there is a 6 dB boost in acoustic level at the microphone. Additional benefits include uniform frequency response even as a sound source is moved around the room relative to the mic’s location. Such movement also does not cause any off-axis coloration, as would be the case with a typical cardioid microphone. This is especially helpful when recording large ensembles such as orchestras, choirs, or theatrical productions.
In the studio
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The Audio Technica U841 boundary mic taped to the control room window for background vocals or hand clap recording.
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It’s not too great an overstatement to say that there are few, if any, instruments a PZM cannot accurately capture. The biggest challenge is realigning your thinking to make the most of the PZM’s boundary recording characteristics. A traditional cardioid mic has a heart-shaped pickup pattern, with the best frequency response in the mic’s sweet spot, directly on-axis to the capsule.
PZMs have an omnidirectional pickup pattern, but rather than a full 360-degrees of sensitivity, the mic’s boundary plate reduces the mic’s pickup area to a 180-degree hemisphere. Therefore, PZMs are normally placed on the floor or taped to a studio wall, doo,r or window. A boundary surface, such as a studio floor, extends the mic’s own boundary plate resulting in a larger hemispherical pickup area and extended low frequency response.
“I recently had a singer songwriter in the studio, and on one particular song, I suggested we add a stand-up bass,” explains Jeff Crawford, a veteran studio and broadcast engineer (and regular PSE contributor). “Since we would be overdubbing the bass part to her existing tracks, I used a Cascade Fathead ribbon as my close mic on the bass, about two feet from the instrument, which gave us a thick, warm sound. Then I put a pair of Crown PZMs on the studio hardwood floor about six feet away from the bass, separated from each other by about 8 feet. I centered the Fathead and panned the PZMs hard left and right in the mix. The result was a very rich, wide bass sound that had a lot of natural room ambience from the PZMs blended with the Fathead’s warmth. The PZMs not only deliver the room tone, they faithfully reproduce the high end overtones of the bass, lending the track a nice realism.”
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Cascade's Fathead series of ribbon mics provide excellent response at a modest price.
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Jeff also reaches for his PZMs whenever a group that includes acoustic piano is playing live in the studio.
“Leakage into a piano that has the lid up even a tiny bit gets to be a problem if you have to do any fixes or overdubbing afterwards. I’ll tape a pair of PZMs inside the piano lid, one over the bass strings and the second to pickup the high end of the instrument. I don’t place them directly over the hammers, but further back. Once again, I’ll pan the mics hard left and right, then lower the lid all the way down and cover the piano with packing blankets to keep out leakage. With this set up, the rest of the band basically disappears from the piano mics. PZMs also accurately capture the high transients of the hammers hitting the strings.”
Drums are another candidate for PZM capture, however, since the mics are omnidirectional, one has to rethink how to use them effectively.
“In one of my previous studios,” continues Jeff, “I had sound panels hanging down from the ceiling and I sometimes would tape my PZMs to them above the drummer to use as left and right overheads. The clarity and realism is stunning when you use them this way. Of course, if a full band is playing in the studio, your are going to be picking up all the other instruments in the overheads, so you have to decide how much leakage is acceptable.” [Ed. Note: For overhead use, you must mount the PZMs on a larger panel, at least one square foot each, to insure that the boundary principle will be in effect.)
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Two PZMs taped under the piano lid deliver a clean, present sound and all but eliminate leakage from other instruments in the room. |
Since PZMs can handle very high SPLs (up to 150 dB), they are ideal for high energy drummers who have a very loud sound output. Record a kick drum by placing the PZM on a hard surface in front of the kick for a rich full sound and even room ambience. Adjust the balance between the direct sound of the kick and the room tone by moving the PZM closer or further from the sound source.
If you are recording drums alone, you can also experiment with simply placing a PZM about two feet to the left and another to the right of the drummer and panning them. You’ll get plenty of punch and room ambience, along with a wonderful stereo spread. Some engineers have even taped a PZM to a drummer’s chest with good results, providing a real “drummer’s perspective” to their recording.
Old-school vs. new-school
Jeff and I decided to try a solo acoustic guitar recording with one of his vintage Crown PZM-30 GPs and a new Audio-Technica U841A boundary mic. The U841 has a street price of $230, while the PZM-30D (Crown’s modern replacement for the PZM-30 GP) sells for about $270. I tuned up my Taylor “Big Baby,” and we set up the two PZMs about four feet in front of me, placing them on the studio hardwood floor.
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Audio Techinca's U841 is an excellent all purpose boundary mic. |
Mic placement has never been easier. One of the first characteristics of PZMs, their stunning realism, was apparent as we simply checked levels to see that both mics were hot. As Jeff played back my recorded voice, it literally sounded as if I was in the control room standing right next to our monitors. This is due to the PZM’s uncolored pickup of reflections around the room as sound is propagated.
For the first test track, I played a driving rhythm part with a lot of percussive elements. Both mics picked up the guitar nicely, with a full tonal balance. As we listened more closely, we noticed that the U841 accentuated the guitar’s upper register, while the Crown seemed to deliver a slightly smoother upper end. Jeff pulled out the frequency response graphs for the two mics and why we were hearing a difference in tonal quality became apparent.
The U841 features a boost that starts around 4K and begins to taper down around 7K. That really pumped up the Taylor’s brightness. The Crown has a much smaller boost that peaks closer to 10K, well above the guitar’s fundamental frequencies. [Ed. note: The current production Crown PZM-30D mentioned above offers 20 Hz – 20 kHz response, as well as selectable flat or rising high frequency response.]
Practically speaking, the U841 delivered an acoustic guitar sound that would likely fit into a full band mix nicely, adding sparkly highlights and making it easy to hear the acoustic track in the midst of a full band without having to crank up the track. However, for solo guitar recording, the vintage PZM-30GP offered a bit more warmth that sounded more natural to both of us.
We next tried out some hand percussion, as well as hand claps, hand drumming, and foot stomping on the studio’s hardwood floor. Both PZMs proved their ability to reject any rumble or low frequency thumps, which can be a problem with other types of microphones. (Yet another application for PZMs is for use on tables and lecterns, where unwanted mechanical noises can be especially troublesome.) PZMs, by their design, are only sensitive to acoustic transmission through the air, not mechanical pickup through the mic’s housing. The ability of each mic to capture the sharp transients found in percussion recording was superb, another PZM trait.
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The Crown PZM-30D, pictured here, is the modern day replacement for the PZM-30GP used for this session.
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Paralleling the response we heard on the acoustic guitar, the U841 enhanced the snap and presence on the hand claps and hand drumming. By comparison, the Crown delivered a slightly smoother, less present sound. For such applications, either PZM would be more than satisfactory, with a slight amount of EQ applied if more or less high end response would better suit the mix.
“The PZMs also work well for bongos or congas,” stated Jeff, “I’ve taped one to the back of the control room door and played bongos in the control room about a foot away from the mic. Set up took a few seconds and the sound was perfect.”
Jeff also recalled using a PZM for a vocal effect. “I was recording a group and we set up a PZM on a mic stand with no additional boundary. This has the effect of reducing low frequency sensitivity. Then we had the background vocalists cut their tracks and we got a very bright, sizzling sound, because the lyrics had a series of sibilant consonants. I used it as a panned effect and it really worked well. Later, I tried to duplicate that sound using a regular mic and I couldn’t get the same sizzle, the PZM was perfect for such a vocal effect.”
In summary, PZMs are rugged, durable, and extremely versatile mics – adding a pair to your studio’s mic locker certainly seems like a good investment for expanding the range of what you can do. The only real limits to creative uses of these neglected mics are your own imagination. |
