Part XVIII: Combination microphones

5.7. Electrical combination of pressure and pressure-gradient transducers

One possible way of producing a directivity pattern that has a single maximum (so-called unidirectional characteristic) is to combine electrically the outputs of a pressure and a pressure-gradient microphone. The two units must be located as near to each other in space as possible so that the resulting directional characteristic will be substantially independent of frequency.
Microphones with unidirectional, or cardioid, characteristics are used primarily in broadcast or public-address applications where it is desired to suppress unwanted sounds that are situated, with respect to the microphone, about 180   degrees from wanted sounds. ...

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