Modern electret applications-the first 20 years

A review is presented of commercialization of the foil electret microphone. The success of these microphones is attributed to their excellent acoustic properties, low cost, mechanical simplicity, and above all the stability of the charge in Teflon even under extreme environmental conditions. The author discusses the No-Voltage Velotron, the application of thick wax electret microphones in Japanese field equipment during World War II, and the development and application of thin piezo- or pyroelectric polymer electrets. Since the commercial success of electroacoustic transducers in the audio frequency range, microphones using thin polymer electrets have been proposed that extend the frequency range from 10^-3 Hz to more than 2×10⁸ Hz for use in solids, liquids, and gases. The physical size of microphones based on both of the above principles have been reduced to micrometer dimensions