Piezoelectret microphones.

Abstract

Piezoelectret microphones, a recently described new class of electret microphones, are based on the strong piezoelectric effect of internally charged, cellular polymer films. Such piezoelectret films have a large number of internal voids within the polymer. The upper and lower walls of these voids are oppositely charged. Films of this kind, when metallized on both sides, show high-piezoelectric activity and can be directly used as microphones. Recent work on these transducers has concentrated on stacking of piezoelectret films to improve sensitivity, on directional microphones, and on investigations of new film materials with improved charge stability at elevated temperatures. Stacked transducers have sensitivities proportional to the number of piezoelectret layers. With six layers, values of approximately 15 mV/Pa can be achieved. Directivities include omnidirectional, bidirectional, and cardioid patterns as well as dimensional characteristics. The new materials are based on fluoropolymers. These are, as opposed to the originally used polypropylene, not available in suitable cellular form. However, useful films can be made by fusing thin fluorocarbon films such that an array of small air voids is created between the layers. Transducers consisting of charged films of this kind withstand temperatures of 90 °C without significant loss of sensitivity.