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Technology Showcase: Boundary Microphones

Apr 9, 2009 12:00 PM, By Bennett Liles

From conference rooms to stage performances, the applications for boundary microphones are numerous.

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Peavey PSM 3

Peavey PSM 3

In the late ’70s, experiments in sound recording showed that within a few millimeters of a hard, flat surface, sound waves coming from different locations are in-phase due to the near stop of air molecules in the area near the surface. These cause a slowdown in the molecules above them, and the effect decreases with distance from the surface. This thin area of slowed waves is referred to as the boundary layer or pressure zone, thus the name boundary microphones or pressure-zone microphones (PZMs). The wave phenomenon results in an increase of acoustic pressure of about 6dB. Because of the wave characteristics in this pressure zone, there is little frequency coloration on sounds from different directions and the level of reverberation and tonal quality varies little as the source of the sound moves around the microphone or, in most cases, the table on which the boundary microphone rests.

The applications for boundary mics are numerous, the most common being the conference tables in boardrooms and meeting suites, but they are also used in piano pickup and for voice in situations where the speaking person cannot be right next to the mic. The boundary effect is also used in picking up stage performances by using cardioid microphones laid on foam pads, called mic mice, along the front of the stage, although placement can result in some frequency coloration. More recently, there have been versions developed for permanent table mounting where the microphone is actually screwed into a hole in the table surface. A wide variety of mics of this type has spread rapidly through the industry. Nearly all are condenser types with very good frequency response. In this survey, we will have a look at some examples of each type.

With a cool appearance vaguely reminiscent of a stealth fighter, the C 680 BL from AKG has a cardioid pickup pattern with a frequency response of 60Hz to 20kHz and an A-weighted noise ratio greater than 67dB. The unit operates on 9VDC to 52VDC phantom power and can accept a sound pressure level up to 115dB with 1 percent total harmonic distortion. The C 680 BL comes with a 10ft. cable and a 3-pin XLR connector. The unit’s electrical impedance is less than 200Ω, and it includes a power adapter. The non-reflective black finish is standard, but the unit is suitable for repainting.

The AM-42 boundary microphone from Ansr Audio is an electret condenser type with a cardioid pickup pattern and a three-position low-frequency contour control that extends the versatility of the unit to cover some sound-reinforcement applications. The normal frequency response is flat from 200Hz to 5kHz with a presence bump from 6kHz to 10kHz. Primarily intended for permanent installation, the mic comes with an unterminated cable and a Phoenix connection. The internal impedance is 1,000Ω, and the sensitivity is -65dB with a maximum SPL capacity of 130dB.

Designed for permanent mounting into a conference table or other flat surface, the 220VP condenser boundary microphone from Astatic has the capability of exhibiting a number of continuously variable pickup patterns. The unit is encased in an RF-resistant case and has threads for screwing the body directly into a flat surface up to 3in. thick. The integral 80Hz, 12dB-per-octave high-pass filter removes rumble and room boom, and the design leaves only the smallest structure protruding above the surface. The unit is terminated with a standard three-conductor XLRM connector and includes shock-absorbing polymer bushings.

Audio-Technica ES961RC

Audio-Technica ES961RC

The ES961RC cardioid condenser mic from Audio-Technica features an ultraquiet electronic touch switch, programmable external contact closure for control of external devices, and an LED indicator. There are three modes to which the electronic touch switch can be set: touch on/touch off, touch-to-talk, and touch-to-mute. A recessed switch on the bottom of the microphone offers either local or remote operation of the switch and a third position on the switch provides external control of the LED. It’s equipped with Audio-Technica’s UniGuard RFI shielding to protect against radio-frequency interference. The condenser mic operates on phantom power from 11VDC to 52VDC.

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