Acousto-mechanical pattern changing

A principle of controlling the directivity of a microphone by changing the admittance of sound to the back of a microphone diaphragm, for instance, switching from omnidirectional (only frontal sound, no sound entering the rear) to a cardioid (some sound reaching the backside of the diaphragm). Schoeps has been especially clever using this principle for single-diaphragm microphones.