Electret transducer

(electronics) An electroacoustic or electromechanical transducer in which a foil electret, stretched out to form a diaphragm, is placed next to a metal or metal-coated plate, and motion of the diaphragm is converted to voltage between diaphragm and plate, or vice versa.

A device for the conversion of acoustical or mechanical energy into electrical energy, and vice versa, which utilizes a quasi-permanently charged dielectric material (electret). Examples are certain microphones and headphones. Electret devices are therefore self-biased electrostatic or condenser transducers. They thus exhibit all the advantages of this transducer class, such as wide dynamic range and flat response over a frequency range of several decades, without requiring the external bias necessary in conventional transducers of this kind. See also Electret; Electrostatics; Transducer.

The basic component of all modern electret transducers is a “foil electret” consisting of a thin film of insulating material that has been electrically charged to produce an external electric field. Strongly insulating materials capable of trapping charge carriers, such as the halocarbon polymers, in particular polyfluoroethylenepropylene (Teflon), are best suited for this purpose. Before charging, the material is either metallized on one side or backed up with a metal electrode.

A simple implementation of the most widely used electret transducer is the foil-electret microphone. The diaphragm, typically 0.5- or 1-mil (12- or 25-micrometer) Teflon metallized on one surface, is charged to 100–200 microcoulombs/m² (65–130 nanocoulombs/in.²) corresponding to an external bias of about 200 V. The nonmetallized surface of this foil electret is placed next to a backplate, leaving a shallow air layer. The stiffness of the air layer can be decreased (and thus the sensitivity of the microphone can be improved) by connecting the air layer to a larger cavity by means of small holes through the backplate. The backplate is either a metal disk or a metal-coated dielectric. The electrical output of the microphone is taken between the backplate, which is insulated from the outer case, and the metal side of the foil. The output is fed into a high-impedance preamplifier. See also Microphone.

Compared with conventional electrostatic transducers, electret microphones have the following advantages: they do not require a dc bias; they have three times higher capacitance per unit area, resulting in a better signal-to-noise ratio; and they are not subject to destructive arcing between foil and backplate in humid atmospheres and under conditions of water condensation.

Apart from its use in electroacoustic transducers, such as microphones and earphones, the electret principle has been applied to electromechanical transducers. Examples are touch or key transducers, Korotkov sound pickups, impact-sensitive line transducers, relay switches, and optical display panels.

Because of their favorable properties, simplicity, and low cost, electret transducers have been used often both as research tools and in the commercial market. Among the research applications are microphones for use in acousto-optic spectroscopy, applied to the detection of air pollution and to the study of reaction kinetics of gases and optical absorption of solids. Other applications of electret microphones have been in aeronautics and shock-tube studies. The wide frequency range of electret transducers made possible their application in infrasonic atmospheric studies and in ultrasonic investigations of liquids and solids. In addition, ultrasonic arrays of electret microphones have been used in acoustic holography. Research uses of electromechanical electret transducers have been in such diverse areas as vibration analysis and leak detection in space stations. In these applications, the simplicity and reliability of electret transducers are of importance. See also Infrasound; Photoacoustic spectroscopy; Shock tube; Ultrasonics.

Of all commercial applications of electret devices, the high-fidelity electret microphone for amateur, professional, studio, and cassette recorder use is most prominent, accounting for more than one-half of the entire output of high-fidelity microphones. See also Sound recording.