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Exploring the Boundaries – A Close Look at an Invisible Microphone

By Ken Hamberg

Boundary microphones as a class are often overlooked – literally. Flat-lying and inconspicuous by design, they lack the glamorous appearance and prestige of their conventional large-diaphragm counterparts, which are often photographed in the company of the world's best-known and culturally iconic singers, entertainers and public servants. Boundary microphones in fact enjoy a ubiquitous if highly discreet presence in recording studios, concert halls, installations, public address systems, conference and meeting rooms, and houses of worship. Regardless of their humble, vaguely bug-like appearance, boundary microphones represent some of the most versatile, functional, and reliable mics ever made, and we'd like to take a brief look at how they work and what they can do for you.

On Reflection

Born and bred in the U.S.A., boundary microphones are a relatively recent innovation invented in 1978 by talented audio consultant Ed Long and veteran recording engineer Ron Wickersham. The first marketable prototype was built by Ken Wahrenbrock, an audio design whiz with a gift for the entrepreneurial, and quickly licensed by Crown for manufacture in 1980. The prototype was known as a PZM® (Pressure Zone Microphone) based on the Pressure Recording Process™ developed by its inventors.

PZM's are radically different in design from conventional microphones. Their tiny condenser capsules, which were originally omnidirectional (360° polar pattern), are usually encased in low-profile metal housing designed for placement on hard, flat surfaces like walls, floors, tables and piano lids. They respond best when surface-mounted, and are ineffective when stand-mounted or hand-held.

Crown PZM Microphone

The diaphragm is mounted face-down in the pressure zone directly above and parallel to a reflective boundary plate, secured by a capsule holder. The pressure zone is the area next to the boundary where the direct and reflected sound waves arrive at the same time, in phase. The in-phase signal as it's reflected off the boundary plate surface and picked up by the capsule is doubled in Sound Pressure Level (SPL), showing a 6dB increase in amplitude. The result is high natural gain without increased self-noise, a smooth, flat frequency reproduction with minimal off-axis coloration, and high sensitivity.

Boundary mics by design have very low mass and highly-damped diaphragms, thereby showing great resistance to mechanical vibrations such as surface thump or rumble. They also reproduce room ambience with great natural clarity and brightness while exhibiting consistent frequency response regardless of the direction of the source signal, particularly the omnidirectional capsules.

The sensitivity of a boundary mic may be naturally increased by placing it between multiple boundaries. When positioned in the corner of a room, for example, where the floor and two walls converge at right angles, the mics sensitivity will increase by 18dB, or 6dB per surface without any increase in noise. That's a pretty significant boost of natural gain without touching a knob on the mixer.

As we mentioned earlier, the original PZM's were configured with omnidirectional polar patterns. They're now available in tighter cardioid, hemispherical (half-omni) and supercardioid versions while still exhibiting the same flat off-axis response along with a minimized low-end proximity effect that's common to conventional mics. In the PCC (Phase Coherent Cardioid) boundary design a supercardioid capsule is mounted perpendicular to the boundary plate, aiming forward, as opposed to the parallel placement of the omnidirectional PZM capsule. Technically they're not PZM's, but their attributes are very similar.

Apart from the acoustic benefits of a boundary microphone, you can't overemphasize its discreet appearance and innocuous character. Interviewees, focus groups, lecturers, and even musicians who might otherwise be intimidated by a conventional mic pointed at their faces or instruments often either forget about or are unaware of the presence of a boundary mic while speaking or performing.

Boundaries are easily hidden and are practically invisible on-camera, so they're often used in theatre sets, opera stages, and in television studios. In conference rooms and on the podium boundaries present a clean, unintimidating alternative to a microphone forest, providing greater coverage with fewer mics and fewer distractions. Boundary microphones are available in wireless versions as well, further diminishing cable clutter.

Shure MX690 Wireless Boundary Microphone

Applications

Boundary mics often exhibit better reach, or the ability to reproduce quieter, more distant sounds (including speech) than conventional microphones do. This is because of their sensitivity and higher gain achieved by surface mounting, the wider, smoother frequency response achieved by their remarkable phase coherency, and their greater high- frequency reproduction of reverberant sound.

The clarity and rich high-frequency content of the ambient sound reproduced by boundary mics is also enhanced by the fact that the incoherent and more random reverberant sound is boosted only half as much (a 3dB per surface average) as is the phase-coherent direct sound. Not only does this allow for effective use in studio or concert hall-installed sound applications, it also introduces a bit of wiggle room for less obtrusive placement on the conference room table or the live panel discussion in the meeting room.

Below you'll find a selection of useful tips for using boundary mics in recording and sound-reinforcement applications. Treat them as suggested points of departure – they're microphones, and like all microphones a bit of trial-and-error is always involved for proper positioning.

  • Conference Rooms: These rooms usually are, or should unreflective and minimally reverberant in order to achieve maximum clarity (carpeted, draped, acoustic ceiling tiles, etc.). A single PZM in the center of the table will work well. For longer tables with larger groups, a PZM in the midst of groups of 4-6 people, with no person farther than 3' from the nearest mic should do the trick. An automatic mixer is highly recommended to diminish the noise and signal degradation caused by multiple open mics. Ceiling mounting of the boundary mic is often effective as well.
  • Shure SCM410 Automatic Mixer

  • Lecterns and Podiums: A cardioid or supercardioid boundary on the lecterns top shelf, with the capsule facing the speaker works well. If the lectern has a raised edge, the general rule is position the mic twice as far from the edge as the edge is high. You can also tape an omni to the inside corner wall of the lectern for a bit of that high-gain boundary boost.

  • Altars: Cardioid or supercardioid on the table surface aimed at the speaker, 2-3' away

  • Courtrooms: Cardioid or supercardioid mounted on the bench or witness stand offers intelligibility, freedom of movement, and won't intimidate the witness. Again 2-3' is a workable distance from the subject, centered.

  • The Stage: 2-3 cardioids depending on the size of the stage, a foot from the lip of the stage. 2 mics should be spaced 20' apart, 3 mics around 15'. To reduce pickup from the pit, a 2' piece of 4" thick foam placed roughly an inch behind each mic is recommended. For extra reinforcement, overhead or rear-wall mounted omnis are often used. Omnis at the front of the stage roughly 4' apart often work well as audience mics.

  • Grand Piano: Tape 1 or 2 omnis to the underside of the lid, spaced 8-10" horizontally from the hammers and about 24" from each other. For a different stereo spread, tape 2 omnis under the lid, one over the treble strings close to the hammers, the other over the bass strings well away from the hammers. One will emphasize the hammer attack; the other will reproduce more of the piano's string and body resonance. Use the lids short stick and a sound-proofing blanket or close it entirely to reduce leakage and feedback.

  • Kick Drum: A cardioid inside the drum, positioned on a pillow or other cushion 2-6" from the batter head and aimed at the beater will often sound stupendous, both live and in the studio.

  • Ambient Mics for the Studio: A pair of opposite wall-mounted omnis facing the drum kit, brass and string ensemble, or solo instrument will often impart a pleasing natural room ambience to a recording without the reflective boom often encountered using conventional mics. Placement at or near the corners of two walls can create the illusion of greater size and breadth to the space than actually exists.

  • Small Acoustic or Vocal Ensembles in the Studio: A pair of omnis on the floor 3-5' apart at a distance of 4-8' from the performers.

  • Sporting Events: It's open season for this application. To give you an idea, you'll find omnis mounted on the backboard under the hoop or on the floor at center court at a basketball game, taped to a corner post in the boxing ring or the goal post at a hockey game, and on the back wall at a bowling alley. In fact, there are few sporting events that don't involve boundary mics. Who knew?
  • As you can see, boundary mics are quite versatile and may be used in many more situations than we've covered here, but the list above should give you a good idea of their range and practicality. We should also mention the common use of what are known as PZM boundary panels, which are usually made of plexiglass or other rigid, reflective materials.

    These panels, the design of which was pioneered by Ken Wahrenbrock, the builder of the original PZM prototype, may be used to modify and tailor the directionality and frequency response of a boundary mic. By building single or multiple panel structures, permanent or portable, you can emulate the floor or wall surface and corner boundaries that induce the high natural gain and flat frequency reproduction boundary mics are capable of in their unique way.

    Whether you're involved in conference or courtroom recording, ENG or focus group interviews, studio recording, or stage and studio set production, boundary mics offer an excellent microphone alternative meant to be heard, not seen.

    Below we feature a selection of wired and wireless boundary mics available at B&H, arranged by price from high to low.

    Revolabs Solo Series Shure SLX Wireless Series

    For a list of all products highlighted in this article, click here.

    Please email feedback on this article, or suggestions for future topics, to audiofeedback@bhphotovideo.com.

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