Transducer research at Bell Laboratories under Manfred Schroeder. Free

Electro‐acoustic transducers were one of the research topics in Manfred Schroeders Laboratory at Bell Labs. One of the early highlights was the invention of the polymer‐film electret microphone in 1962. Over the years, the activities were extended to the investigation of appropriate electret materials, to directional microphones, and to microphone signal processing. Manfred himself always had a keen interest in this field and made some notable contributions such as the design of directional microphones based on gradients of various orders with toroidal and unidirectional directivity characteristics. The search for electret materials suitable for electro‐acoustic transducers led to the discovery of the superior charge retention properties of the fluoropolymers. Apart from microphones, other types of transducers such as touch actuators, transducer arrays for acoustic holography, and sensors for the detection of acoustic ion waves were investigated. Much of the later work of the author, performed at the TU Darmstadt, was based on the results of the early studies at Bell Labs. Examples are the introduction of silicon‐based condenser microphones made with the methods of micromachining, using electret or external biasing, and new types of piezoelectric transducers consisting of charged cellular polymers, so‐called ferroelectrets.