New developments in the field of microphones or acoustic sensors, based on the silicon, optic-waveguide, and polymer technologies, are discussed. The silicon sensors are produced on silicon wafers with micromechanical methods. Subminiature transducers of this kind, utilizing the condenser, piezoelectric, or piezoresistive principles, are made with membranes of about 1 mm2 area and less than 1 µm thickness. Active silicon microphones based on the direct modulation of the drain current in a field-effects transistor are also under study. Optic-waveguide sensors employ glass fibers or waveguides on glass or polymer substrates and detect phase or intensity modulations of the transmitted light caused by the sound waves. Polymer sensors utilize polymer electrets or piezopolymers, usually as thin membranes or layers. Polymer electret sensors find widespread applications as microphones and ultrasonic transducers. New developments include the use of electret biasing in silicon sensors and the design of electret-microphone arrays. Piezoelectric polymer sensors are being improved by the use of better materials and by innovative poling techniques, such as monomorph poling. Also of interest are hybrid sensors using silicon structures covered with piezoelectric polymer layers.
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