A Surface Microphone (Boundary Layer Microphone) is a special type of microphone designed to be used attached to a large, flat surface, such as a wall, floor, or table.

This microphone has the capsule (the diaphragm) mounted millimeters from a plate, which in turn is placed on the surface.

In this position, the direct sound wave from the source and its first reflection on the surface reach the capsule almost simultaneously, in phase.

This in-phase summation results in two major advantages:

1. Increased Sensitivity: The captured signal is 6 dB stronger, which means a cleaner sound with less preamplifier noise.

2. Elimination of the Comb Filter (Comb Filtering): Prevents frequency cancellation (the "hollow" and "phased" sound) that occurs when a conventional microphone captures the direct sound and surface reflections at slightly different times.

PZM (Pressure Zone Microphone)

The Surface Microphone is popularly known by the trademark PZM (Pressure Zone Microphone), which has become synonymous with this type of microphone. They are widely used to capture sound in conference rooms, theaters (placed on the stage floor), or to mic the inside of a bass drum.