Boundary microphone

Boundary mics have a relatively flat profile, they have the advantage of being less visually obtrusive, such as when they are placed in a conference table or the floor or a musical theater stage.

At the same time, the placement of the mic on a table or floor may increase the likelihood of unwanted sounds from contact of items or body parts on those hard surfaces.

On August 8, 1904, he delivered a groundbreaking paper, Über Flüssigkeitsbewegung bei sehr kleiner Reibung (On the Motion of Fluids in Very Little Friction), at the Third International Mathematics Congress in Heidelberg.

In a study of flush-mounted microphones, they realized if the mic is only a few millimeters away from a large surface, there is a boost in coherence of sound signals, as they are still in phase after being stopped at the boundary.

The direct and delayed reflected sounds will combine at the microphone to create comb filtering, with constructive and destructive interference causing peaks and valleys in the frequency response.

Radio Shack and Tandy also use the PZM term for their particular design of a boundary mic, in which the condenser capsule was positioned slightly higher than the base plate.

Custom painting can be used to provide an exact color match with a wall, in cases where the building architects wish there to be minimal visual impact (e.g. a historical church or a Classical music stage).

On the other hand, a boundary mic intended for use on a music theater stage is much more likely to face impacts (e.g. being stepped on), so it needs a robust, thick steel casing.

Gold Line's CBM1 boundary microphone and Crown's "Sound Grabber II" have an internal battery to power the condenser.

As shown on manufacturers' datasheets, the mic capsule retains its directionality (cardioid or supercardioid polar pattern) and prevents comb filtering by keeping phase interference above the audible range.

Boundary mics intended for boardroom use may have a range of additional features that are helpful in a conference setting, such as an "on" indicator LED, an on-off switch (or a "push to talk" or "mute" button), and wireless capability to allow easy repositioning during the event.

Boundary microphone (Audio-Technica ATM87R)
Two examples of different boundary mics (top) and a diagram of the elements of a boundary microphone. The diagram shows the location of the mic capsule, the plate, the aperture, and the pressure zone.
A Crown Audio brand PZM, the "Sound Grabber II". It is shown with adapters to enable it to be connected to different mixers and audio devices. The mic has an internal battery so that it can be used with consumer audio devices that do not supply phantom power.