ACOUSTIC SENSORS

An overview of new developments in the field of microphones or acoustic sensors, based on silicon, optical-waveguide and polymer technologies, is given. The silicon sensors are produced on silicon wafers by micromechanical methods. Subminiature transducers of this kind, utilizing the condenser or piezo-electric principles, are made with membranes about 1 mm2 in area and less than 1-mu-m thick. Other micromechanical sensors based on sound-pressure-induced modulation of the drain current in a field-effect transistor are also under study. Optical-waveguide sensors are either made of glass fibers or of waveguides on glass or polymer substrates and detect phase or intensity modulations of the transmitted light caused by the sound waves. Acoustic sensors of this type are already in use as hydrophones and are presently under study as microphones. 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. Hybrid sensors using silicon structures covered with piezoelectric polymer layers are also of interest.