Transducer research at Bell Labs 1975 and beyond

Abstract

Condenser microphones, invented at Bell Labs by E. C. Wente and reported in the literature in 1917, are well known for their flat frequency and phase response over a broad range, which make them useful whenever high-quality sound measurements or recordings are required. Two drawbacks, the required high-dc bias and the cost of construction, prevent them from being applied in commercial products such as communication devices, toys, hearing aids, and sound-recording devices. The electret condenser microphone (ECM) eliminates both of the above drawbacks by replacing the needed dc bias with a thin polymer film that is given a permanent charge, the electrical analog of a permanent magnet. Cost of construction is reduced mainly because there is no need to guard against high-voltage breakdown. This technology enables the construction of one- and two-dimensional arrays and up to third-order differential systems for both near- and far-field applications. ECMs are also pliable to most any shape or size, so the construction of continuous strip arrays, transducers for cardiovascular monitoring, MEMS devices, and touch sensors becomes practical. We will discuss some of the experimental systems developed at Bell Labs.