BIDIRECTIONAL SENSITIVITY PATTERNS
A bidirectional microphone is a noise-cancelling microphone (such as the Sennheiser HD 414 close-talking microphone that is so popular in speech research).

A bidirectional microphone uses properties of a gradient microphone to achieve noise cancellation. Sound pressure never arrives at the front and the back of the microphone at the same time. However, noise, which arrives from the side, does, and hence is cancelled. These microphones have directional sensitivity patterns: Such microphones are very sensitive to placement, and cannot be used interchangeably with recognition systems (microphone independence?).