Second-order gradient noise-cancelling microphone

A piezoelectric, second-order gradient, noise-cancelling microphone has been developed for an advanced narrow-band digital voice terminal (ANDVT). It has much greater capability to reject acoustical noise from distant sources than does the conventional first-order type. Precise balancing of the phase and amplitude response of the acoustical components is required. Four sound ports are arranged in a square configuration, providing a second-order characteristic and 4- leaved-rose polar response. The lowest resonance frequency is 20 kHz, well above the speech band. There are no damping components which tend to compromise acoustical symmetry. The noise-cancelling performance conforms closely to theory.