Outstanding contributions to the design of electroacoustic transducers and, most notably, the co-inventorship of the foil electret microphone and of the silicon condenser microphone
Pioneer of Microphone Technology
The quality of every true-to-original transmission of speech or of a voice signal, respectively, is chiefly determined by technical peculiarities of the microphone. The physical principle of operation of the electroacoustic transducer, its technical construction, especially of the microphone capsule itself, all have an enormous impact on the quality of the analog electric signal representation. Moreover, of course, optimal positioning of the microphone (possibly within an array of microphones) relative the acoustic source plays an important role, too.
Every communications engineer has at least an idea of how to achieve acoustic-to-electrical conversion. A microphone is being used to convert sound energy (air vibrations) into mechanical energy (membrane vibrations) and, finally, into time-continuous electrical signals. There are, however, a variety of specific designs for microphones some of which are less well known. The more common designs include carbon microphones, externally polarized condenser microphones, prepolarized electret condenser microphones, magnetic microphones, and piezoelectric microphones. Depending on the construction of the microphone capsule, pressure sensing devices are to be distinguished from differential pressure sensing devices. Finally, the diameter as well as many other details of the mechanical design of the capsule determine the microphone’s angular resolution with respect to sound signals impinging from different directions. Some microphones exhibit almost ball-shaped (omni-directional) sensitivity patterns whereas others are specifically built to have kidney-shaped characteristics so that they can conveniently be used by two adjacent speakers. On the extreme, cardioid or even super-cardioid microphone designs are possible that are especially sensitive along the main axis of the microphone and thus demonstrate perfect spatial resolution.
The diaphragm of a condenser microphone is formed by a thin metallic plate. The opposite plate consists of a metallic block with a drilled hole of appropriate diameter in it. According to a basic law of physics, the capacitance of such a specially designed capacitor is inversely proportional to the distance between the two plates. However, to work as an acoustic sensor, the condenser needs a high DC bias voltage. It is connected in series with a high-ohmic resistor. Variations from the bias voltage are proportional to variations of the capacitance of the device.
In the early sixties, the award winner and his former colleague at Bell Laboratories, Jim West, jointly invented a special type of condenser microphone, now known as electret condenser microphone. Following their design, one or both of the two condenser plates are formed by electrostatically pre-polarized material. Thin, extraordinarily strong plastic films of Mylar, or later on, of Teflon materials were utilized in their trailblazing design. In a pre-processing step, these materials are to be heated up and exposed to a strong electrostatic field. Then, after cooling down and under normal operational conditions, the polyester film’s electrostatic polarization is permanently conserved over many decades. Predictions are that the electrostatic charge of these polymer electrets can be sort of “frozen” for at least 200 years. Consequently, in comparison with standard condenser microphones, operation of electret condenser microphones now became possible without the need for an external ‘phantom’ fed DC bias voltage.
By the way, the term ‘electret’ was already coined in 1885 by the English physicist Oliver Heaviside. The name ‘electret’ points out the electrostatic analogy to the permanent magnet. Electret condenser microphones do need very low supply currents, too. These currents are in the milliamp range only and are required to operate an impedance converter. The acoustic sensor is connected to the high-impedance input of a field-effect transistor. Both components are then jointly built into the microphone capsule.
The invention of the electret condenser microphone by Sessler and West dates back 45 years. Its superiority, in comparison with other microphone designs, is based on its tininess and flat amplitude response over the full acoustical frequency range. Nowadays, due to their low sensitivity with respect to undesired vibrations and, most importantly, due to their low production costs, electret microphones are produced in huge quantities. Current production numbers of more than 2 billion devices per year speak for themselves. That technology has clearly stood the test of time. Today, electret condenser microphones make up roughly 80 to 90 percent of the world market. They are applied in a plethora of different ways and in various domains. Typical standard applications include cell phones, movie cameras, voice recorders, hearing aids, sensor applications in automotive industry, and simple acoustically controlled toys for kids.
Over his long career, this year’s Technology Award winner, Gerhard Sessler, has significantly contributed to many more scientific areas. In the decades following the invention of the electret condenser microphone, he earned widespread respect for his work on various topics in technical acoustics, for his investigations into the physical properties of electro-active materials, and for his achievements in digital signal processing of acoustic signals. Ever since his time as a Scientific Staff Member and later on (after 1967) as a Supervisor in the Acoustics Research Department of Bell Laboratories at Murray Hill, he developed an extraordinarily high number of ingenious ideas. Since he became a professor of electroacoustics at Technical University of Darmstadt in 1975, many of his students have been actively involved in these investigations.
Another one of his milestone achievements also aimed at the construction of subminiature microphones. It dates back to his time as a University Professor at the Technical University of Darmstadt in the early eighties. He and his research team then developed the first condenser microphone based on silicon micromachining. It was patented in 1983 by himself and his former PhD student Dietmar Hohm. Gerhard Sessler’s contributions to microphone technology received special attention in 1999 when he became an inductee of the U.S. patent office’s National Hall of Fame. So far, he has been the only German citizen alive who received this highest recognition of excellence and achievements as an inventor.
The Eduard Rhein Foundation honors Professor Gerhard Sessler as an internationally renowned distinguished inventor, as a leading acoustics technologist, and as an outstanding educator. Over many decades, he relentlessly contributed to the advancement of microphone technology and to the better understanding of the properties of electro-active materials. As a talented university educator, he made the fascinating science of electroacoustics accessible to generations of students.
Prof. Dr.-Ing. Horst Bessai,