boundary-layer-direc

Boundary-Layer Technique
BLCg

For CCM Compact Microphones or MK Capsules with KC active Cable (e.g. MK 4 / CCM 4 cardioid or MK 41 / CCM 41 supercardioid).

Turns any directional (e.g. cardioid or supercardioid) CCM Compact Microphone or MK capsule with Active Cable into a boundary-layer (PZM) microphone. This is especially useful when recording moving singers/ actors on a stage, or when a microphone on a stand would be obtrusive. A rubber ring built into the bottom of the plate prevents inadvertent rotation of the plate or marring of the surface on which it is placed.

Diameter: 100 mm
Ssurface finish: matte gray (g)

A directional boundary-layer microphone tends to "even" the balance of speakers and singers at various distances. For sources at greater distances, the sensitivity of the microphone increases in a compensating manner due to the angle of sound incidence, as shown here with a supercardioid.

Boundary-layer recording is a well-known technique by now, offering unobtrusive microphone placement possibilities (no stands or hanging fixtures required, and it is not necessary to aim the microphone) and a directional characteristic that is independent of frequency. It is based on an effect in physics by which sound pressure doubles along a sound-reflecting surface. If a transducer is placed upon (or within) a large, sound-reflecting surface, its output voltage will be twice that of an equivalent transducer in a free sound field.

In theory an ideal transducer of this type would have a membrane of infinitesimally small size, and would be mounted flush with an infinitely large, perfectly sound-reflecting surface. In practice, the plates in which the small transducers of the BLM 3g and the BLM 03Cg are mounted assure reflection of only the high and medium frequencies.
To reflect sound at lower frequencies (greater wavelengths), the dimensions of the surface must be correspondingly larger. So the microphone should be placed on a floor, wall or ceiling, or mounted on another surface large enough to reflect the lowest frequencies of interest.

Ordinary pressure transducers show a difference in sensitivity between direct and diffuse sound at high frequencies. In a properly-placed boundary-layer microphone, however, this difference occurs at all frequencies. The sound pressure for direct sound is doubled at an acoustic boundary (6 dB increase), while the essentially random phase relationships of reflections in the diffuse sound field cause the boundary-layer microphone's sensitivity to increase by only 3 dB. Thus a boundary-layer microphone will suppress diffuse sound somewhat, even though it is not directional. It will pick up less “room sound“ and the recording will be slightly “drier“ than it would be with an omnidirectional microphone.