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Gerhard M. Sessler: Scientific Contributions

Gerhard Sessler contributed extensively to the field of electroacoustic science and engineering. His main activities were in the area of electroacoustic transducers, particularly microphones, where he and his co-workers and students were responsible for some innovations that occurred during the past forty years. These include the polymer-electret microphone and the silicon condenser microphone which are nowadays the dominant microphone types worldwide. Sessler also contributed significantly to research on directional microphones. Apart from transducer engineering, he and his co-workers advanced many areas of research related to transducer materials, nowadays called electro-active materials, such as polymer electrets, piezoelectric polymers, and charged silicon compounds. In addition to these activities, Sessler worked on problems related to the acoustics of concert halls, on the topic of sound propagation in gases and plasmas, on the subject of charges in nonlinear optical glasses, and on other issues. Details of some of these activities will be discussed in the following.

Acoustic Transducers

The invention of the polymer electret microphone, together with J. E. West, originated from work on externally biased, acoustic transducers utilizing polymer films, which they did at Bell Laboratories in the early 1960s. The two inventors noticed that certain polymer films, when exposed to a dc-bias, acquired a more or less permanent charge and became electrets. When used as a membrane in a condenser microphone, the external bias voltage usually needed in these transducers is expendable and a much simpler device, which they called foil-electret microphone, resulted (Sessler and West, 1962 and US patent Nr. 3,118,022, filed 22 May 1962). The originally-used polyethyleneterephthalate (Mylar) film was soon replaced by Sessler and West by the electrically superior fluoroethylenepropylene (Teflon) film. The resulting microphones were much better than other types and they were introduced to the market by Sony in 1968 and by many other companies in the years to follow. Their main features are a flat frequency response, low vibration sensitivity, and very low cost. Such transducers, with electrets either used as membrane or as a layer on the back-plate, have been the dominant microphone type since the early 1970’s and are made nowadays in numbers of 2.5 billion annually, which is 80 to 90 percent of the worldwide microphone production. They replaced other microphone types such as the carbon microphone in telephony or the condenser microphone or dynamic microphone in many hi-fi and other applications.

Sessler and West also worked very successfully on various types of directional microphones and on earphones based on the electret principle. They implemented first-order and second-order gradient microphones of very simple design with bidirectional, unidirectional, or toroidal characteristics. The electret headphones suggested by them in the 1960’s, and later on manufactured by a number of companies, were famous for their superior sound quality due to a flat frequency response and an excellent impulse response.

Another basic and successful invention by Sessler and his co-worker D. Hohm at the Darmstadt University of Technology is the silicon condenser microphone, first described by them in 1983 (Hohm and Sessler, 1983; German Patent Nr. 33 25 961, filed 19 July1983). This transducer was the first all-silicon microphone and the first one-chip condenser microphone described in the literature and patented. Sessler and his group at the Darmstadt University of Technology continued work on the implementation of such transducers and also developed other silicon microphone types, such as FET-modulated, optical, and piezoelectric devices and also silicon earphones. The silicon condenser microphone was introduced to the market in 2002. Such microphones are now made in quantities of over 300 million annually and are mostly used in cellular phones. One of their advantages is that they can used as SMD devices since they are extremely small and withstand the heat produced in reflow soldering processes. Another advantage is the low vibration sensitivity of these devices, due to the small mass of the membrane.

Very recently, Sessler and co-workers utilized the high piezoelectric d33-coefficient of charged cellular polypropylene films (see below) to design very simple piezoelectret microphones (Hillenbrand and Sessler, 2004). These transducers consist only of a stack of such films and proper housing and contacting. The shallow air gap necessary in electret microphones is not required in these devices. A new generation of simple and inexpensive transducers is thus on the horizon.

Electro-Active Materials

Next to transducer engineering, Sessler’s second area of major and life-long interest is research on electro-active materials. This started with his work, jointly performed with J. West, on electrets. Their basic investigations into charge stability of polymers resulted in the discovery in the mid-1960s of the supreme charge storage properties of fluoroethylenepropylene (Teflon FEP) which is still the workhorse material in all electret microphones. Later on they used electron beams to investigate charge-storage phenomena in polymer films. A major contribution of the Darmstadt group was the development of the laser-induced pressure pulse method (LIPP method) to investigate the charge, polarization, and piezoelectricity distributions in the thickness direction of thin polymer films with micrometer resolution (Sessler et al, 1982). They applied this method to a variety of polymers and glasses with electret, piezoelectric, and nonlinear-optical properties. The LIPP method is one of today’s leading methods to map such distributions in electro-active polymers and in polymers used for cable insulation (Sessler, 1997). For example, other laboratories have been able to improve the insulation properties of power cables by studying with the LIPP method the dependence of space-charge formation on additives in polyethylene.

In 1999, Sessler established a group working on the new piezoelectric films consisting of charged, cellular polypropylene or other cellular polymers. These so-called ferroelectrets, mentioned above, exhibited piezoelectric d33-coefficients about ten times larger than those of the best known conventional piezoelectric polymer, namely polyvinylidenefluoride (Hillenbrand and Sessler, 2000). In the meantime, thanks to the work in the Darmstadt laboratory and in some other laboratories, an additional factor of three has been gained by expanding (and thus softening) the cellular materials. Also, the Darmstadt group very recently improved the thermal stability of the charges in ferroelectrets by producing layered systems with gas enclosures on the basis of fluorocarbon polymers instead of the polypropylene (Zhang et al, 2007).

Concert Hall Acoustics

The work of Sessler on concert hall acoustics in the 1960s and 70s commenced when Bell Laboratories was asked to perform, under the direction of M. R. Schroeder, an investigation of the acoustics of Philharmonic Hall in New York. Measurement procedures largely based on the use of digital computers were developed and applied to the Hall. Among the results were correlations between objective concert hall parameters, such as the directional distribution of early reflections, and subjective quality. Also important was the discovery of the so-called seat effect, a low-frequency absorption by the rows of seats which affects the perception of double bass, celli and other musical instruments. Later on this work was extended, under the direction of J. L. Flanagan, to other concert halls. Important findings, such as the effect of the stage enclosure on reverberation time, were reported.

Physical Acoustics

Sessler’s contributions to wave propagation in various media are of more basic interest. Very early, he detected and explained free-molecule propagation in highly diluted gases where there are no collisions of the molecules transmitting the sound. He then studied the effect of relaxation phenomena in gases, such as the excitation of molecular rotation or of molecular dissociation, on sound attenuation. Finally he investigated the excitation and propagation of ion waves in plasmas, found dispersion effects, and contributed to the explanation of these phenomena.

Current Research Projects

See the description in the main areas of present research section.

Selected Literature

  • G. M. Sessler and J. E. West, “Self-biased Condenser Microphone with High Capacitance”, J. Acoust. Soc. Amer. 34, 1787 (1962): First paper on polymer electret microphones
  • G. M. Sessler, J. E. West, R. Gerhard-Multhaupt, and H. von Seggern, ”Nondestructive Laser Method for Measuring Charge Profiles Irradiated Polymer Foils”, IEEE Trans. Nucl. Sci. NS-29, 1644-1649 (1982). Introduction of LIPP method.
  • D. Hohm and G. M. Sessler, “An Integrated Silicon-Electret Condenser Microphone”, Proc. 11th Internat. Congr. on Acoustics, Paris 1983, Vol. 6, pp. 29-32 (1983). First paper on all-silicon condenser microphones.
  • G. M. Sessler, “Charge Distribution and Transport in Polymers”, IEEE Trans. Dielectr. and Electric. Insulat., 4, 614-628 (1997). Modeling and measurements on electret charges in irradiated polymers.
  • J. Hillenbrand and G. M. Sessler, “Piezoelectricity in Cellular Electret Films”, IEEE Trans. Dielectr. and Electric. Insulat., 7, 537-542 (2000). Basic modeling and measurements for cellular films with high piezoelectric d33-coefficient.
  • J. Hillenbrand and G. M. Sessler, “High-sensitivity piezoelectric microphones based on stacked cellular polymer films”, J. Acoust. Soc. Amer. 116, 3267-3270 (2004). First paper on microphones with stacked piezoelectric films.
  • X. Zhang, J. Hillenbrand, and G. M. Sessler, “Ferroelectrets with improved thermal stability made from fused fluorocarbon layers”, J. Appl. Phys. 101, 054114-1 to 054114-8 (2007). First paper on ferroelectrets with high thermal stability and very large piezoelectric d33-coefficients.

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