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Boundary Mics and Automatic Mixers

Sep 15, 2010 12:12 PM, By Mark Johnson


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Automatic mixers and boundary mics go hand in hand (or at least XLR to XLR). Boundary mics can be used in many of the applications where you would also want to employ an automixer—situations where it might not be practical to use a full-time system operator, especially in meeting places such as a conference space, boardroom, or courtroom. Other typical uses can even include houses of worship. As you’ll see, many of the manufacturers produce mics as well as mixers. And there are more choices than you might imagine with quite a few options available for styles and types and, for automatic mixers, performance algorithms.

Boundary Layer Microphones: The Basics
One of the biggest advantages of a boundary microphone design is the reduction of comb-filtering effects. This occurs by greatly increasing the ratio of direct sound versus reflected and ambient sounds the mic picks up. This is accomplished by placing the mic element near a boundary surface such as a floor wall or tabletop.

As a contrast, consider a typical stand-mounted microphone we’re all so familiar with. For distance-miking applications, the sound will arrive at the mic element via two different paths: direct sound and sound reflected off a surface such as a wall, floor, or tabletop. The path of the reflected sound is longer than the direct sound, so the reflected sound arrives slightly later and is delayed at the microphone. The result of the delayed sound combining with the direct sound is called “comb filtering” because when it’s displayed on an analyzer, the response looks like the spiked teeth of a comb.

Boundary layer micro­phones are designed to take advantage of a fixed surface such as a wall, floor, or ceiling. By positioning the mic element close to the boundary, the reflected sound entering the mic is minimized.

Boundary mics can come in a few different form factors. One implementation is small condenser elements that protrude just slightly over the surface of a table, lectern, ceiling, or wall, facing outward. These are also called button mics because of their resemblance to small buttons. Other versions place the mic element just above and perpendicular to the boundary surface or just above and parallel to the boundary surface and facing back to it, also called a Pressure Zone Microphone (PZM). Ron Wickersham and Ed Long were granted a patent for the PZM in 1982, which was initially developed by Ken Wahrenbrock and later manufactured by Crown. The common denominator in all three implementations is that the mic element is just above the boundary surface, thereby minimizing the reflection path.

AKG Acoustics C 547 BL

AKG Acoustics C 547 BL

Another advantage of such mic designs is a low, unobtrusive profile that keeps sightlines clean. Boundary mics employ omnidirectional or cardioid pickup patterns and are well-suited for area-miking on a stage or picking up people’s voices around a conference room table or at a lectern. Boundary mics have also found favor as a kick drum mic, positioned just in front of (or inside) a bass drum.

For practical reasons, many boundary mics are affixed to a small plate; however, the low frequency response improves as the size of the boundary increases.

There are dozens of boundary mics on the market in a variety of price ranges. Following is a sampling of some of the boundary mic models available, listed alphabetically.

Audio-Technica ES961

Audio-Technica ES961

The AKG C 547 BL has a hypercardioid pickup pattern. It features a low-frequency roll-off filter to help reduce foot noise in live stage applications. The 7.3”x4.7”x0.8” (LxWxH) body is designed to be crush-proof and can withstand the weight of an average-sized person stepping on it. The matte black body surface is paintable to blend in with any décor. Its stated frequency response is 30Hz to 18,000Hz. It’s also offered as the C 547 BL-T, a nonwindscreen version designed for tabletop use.

Astatic 930VPL

Astatic 930VPL

The Astatic 930VPL features a continuously variable polar response that allows the contractor to set the pickup pattern to meet the specific need of any particular installation. Features include a dual-output connector design that allows the cable to exit the rear or the underside of the microphone, a programmable membrane switch, and a logic interface for interfacing with automatic mixers, teleconferencing systems, and control systems. The frequency response is 40Hz to 15kHz.

The Audio-Technica ES961 has a frequency response of 30Hz to 20,000Hz and features a half-cardioid polar pattern—cardioid in the hemisphere above the mounting surface. Optional, interchangeable omni and hypercardioid elements are available. Features include a PivotPoint rotating output connector and a heavy die-cast base with silicon pads to prevent surface vibration coupling. The mic is also offered as the ES961RC version with an external contact closure that lets users control a local or remote device from the switch on the microphone.

Audix ADX60

Audix ADX60

The Audix ADX60 features a hemi-cardioid polar pattern with a frequency response of 50Hz to 18kHz. Construction includes a die-cast zinc casing and a steel mesh grille with a low-reflectivity matte black e-coat finish and mini-XLR connector. The mic has a standard cardioid response, but interchangeable omnidirectional and hypercardioid capsules are also available. Specs include a stated frequency response of 50Hz to 18kHz.

Designed for use with with standard videoconferencing systems, the Beyerdynamic Stegos TB wireless boundary mic features a semicardioid pickup pattern, an illuminated multifunction button (manual or push-to-mute), and a maximum operating time of 14 hours from its rechargeable NiMH batteries. It operates in the 2.4GHz range, and its Spread-Spectrum and 128-bit encryption keying guard against unauthorized listening.

Galaxy Audio BN-273PH

Galaxy Audio BN-273PH

The button-style omnidirectional condenser Clockaudio C003E mic features through-desk mounting and a satin black or matte white finish. The threaded brass body has an overall length of 90mm and accommodates mounting surfaces up to approximately 2.75in. thick. Its frequency response is 20Hz to 20,000Hz. Clockaudio’s C004E is a similar model, but it has a cardioid response and a 100Hz to 16kHz bandwidth.

The stylish DPA Microphones BLM4060 features an omnidirectional hemisphere pickup. It’s constructed of a 4in. diameter stainless steel base with a center black rubber disk that effectively shock-mounts the condenser capsule and eliminates vibration noise. The mic’s response is stated as 20Hz to 14kHz, with a +7dB soft boost at 8kHz to improve vocal intelligibility.

Sennheiser e 912

Sennheiser e 912

Part of Electro-Voice’s PolarChoice line, the Boundary Satellite mic features a user-selectable polar pattern (omni, cardioid, supercardioid, or figure-8) and accepts an Electro-Voice or Telex bodypack transmitter for wireless operation. An onboard mute switch can be set to operate as either latching on/off or momentary push-to-mute/push-to-talk, and a high-visibility blue LED clearly displays mic status to the user. Its frequency response is 50Hz to 20,000Hz.

Galaxy Audio’s BN-273PH features a cardioid pickup pattern with a 30Hz to 20kHz response. Recessed DIP switches on the mic’s underside enable users to reset the operation on the top panel on/off button, which is selectable from push-to-talk, push-to-mute, or latching on/off modes, along with a switchable high-pass filter. The BN-273PH has a tough, die-cast zinc body that’s available in black or white finish, and a top-mounted LED glows when the mic is active.

The Sennheiser e 912 features a half-cardioid pickup pattern and a frequency response of 20Hz to 20,000Hz. The e 912 is optimized for instruments, vocals, and speech, and it is available in black or cream. Also offered in gray, the speech-optimized e 912 S is similar, but it features an integrated membrane switch and an internal DIP switch bank providing the following features: on/off, push-to-talk, push-to-mute, or permanently switched on. Additionally, the frequency response of the e 912 S can be changed via a second DIP switch bank, offering low-cut, low/high boost, low boost, or linear.

Shure MX396

Shure MX396

Shure’s MX396 multi-element boundary microphone is offered in MX396C/DUAL (dual-element) and MX396/TRI (tri-element) versions that provide a variety of pickup configurations for end-of-table (300-degree), center-of-table (360-degree), or cross-table (bidirectional) coverage patterns. Both feature an individual audio output for each element, a bicolor status indicator, a programmable mute switch, and a logic input/output for remote LED and mute control. Its frequency response is 50Hz to 17,000Hz.



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