Electret Transducers Research Articles

Electret transducer with a selectively metalized backplate

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Electret transducer with variable electret foil thickness

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Electret transducer with variable effective air gap

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Continuous strip electret transducer array

A rough surfaced backplate has deposited thereon a metallic electrode having a plurality of large areas interconnected by thin strips. The large areas are symmetrically located on opposite sides of the center of the metallic electrode. Furthermore, the distances between the areas is nonlinear. Superimposed on the metallic electrode is an electret foil having a uniform electrostatic charge deposited on the polymer surface thereof.

Electret transducers with variable actual air gap

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An electret touch sensor

In the research presented here we have examined the response of an electret transducer to forces applied over a localized area of the transducer face. The electret is modeled as a membrane under tension suspended above a flat back electrode. The air between the back electrode and the electret foil communicates with the environment through holes in the back electrode. The electret foil is described by a mesh of nodes, and standard finite difference techniques are used to find the deflection of the membrane in response to pressures applied at selected nodes. Static and dynamic analyses of this model have correlated well with the analytical solution for a uniform pressure on the transducer face. The device has been built and its response to impulses at various locations measured. Experimental results and theoretical predictions are in good agreement. Applications for this force‐sensing device are expected to be primarily in the area of robotics.

Electret transducer

Electret transducer

Encapsulated backplate for electret transducers

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Cell assembly for electret transducer

The vibratable diaphragm of an electret transducer has in its central plate portion one or more formed ribs, ridges or projections resting of a substantially flat backplate. These support the active regions of the diaphragm with precise spacing from the backplate. Elongate projections can be formed to relieve membrane stresses in the diaphragm. The flexural stiffness of the diaphragm is sufficient in itself to provide mechanical stability of the diaphragm in the presence of the destabilizing forces of electrostatic attraction by the backplate.

Silicon-dioxide electret transducer

A new type of electret transducer based on the use of an SiO2 backplate electret is presented. Frequency-response measurements on a prototype device demonstrate the feasibility of the design. Possible improvements and the main advantages of the new transducer principle are discussed.

Adjustment of operating characteristics of a telephone transmitter including an electret transducer

A telephone transmitter including an electret transducer is connected to an electrical circuit mounted in the transmitter housing. A resistor which can be of varying value is also connected to the circuit, the value of the resistor being adjusted to bring the operating characteristics within predetermined values. This is done by applying a sound signal of known frequency to the transducer and measuring the output characteristics of the transducer. This enables the suitable value of resistor to be connected into the circuit. By this means variations arising from varying atmospheric conditions at assembly, and other causes, can be allowed for. The metallized strip or film of the electret is preferably mounted with a very slight Vee shaped formation to avoid tight manufacturing tolerances which occur with a straight, or flat, strip.

An experimental test of acoustical reciprocity calibration applied to thermal transducers in superfluid helium

An experiment will be described which has verified an extension of the reciprocity calibration technique to reversible thermal transducers in superfluid helium. A plane‐wave resonator of circular cross section was capped at both ends by reversible teflon electret diaphrams which contained many razor slits to generate or detect thermal waves (second sound). The resonator also incorporated a thermophone and a dc biased carbon resistance thermometer to set independent upper and lower limits on the amplitude of the temperature oscillations within the resonator. The acoustical resonance data was acquired and analyzed by a computer controlled system described previously [D. V. Conte and S. L. Garrett, J. Acoust. Soc. Am. Suppl. 1 72, S82 (1982)]. The temperature excursions measured by the reciprocity method fell between the upper and lower limits which, for lower modes, were separated by only a few percent. At higher modes the lower limit departed from the upper limit due to the thermal inertia of the resistance thermometer. Temperature excursions smaller than 10−8 °K/[(Hz)1/2] were detectable using the electret transducers and a pre‐amplifier with a noise floor of 10−8 V/[(Hz)1/2]. [Work supported by the NPS Foundation Research Program.]

Response of an edge‐supported, annular, electret earphone

In many electret transducers, the diaphragm is held above a metalized backplate by circular ridges machined into the backplate. Such transducers may be thought of as a series of annular electret transducer segments, each of which has an edge‐supported diaphragm. In the treatment presented here the behavior of an edge‐supported, annular, electret earphone is studied analytically in a two‐step procedure. First, the static diaphragm deflection is determined. A stability criterion is developed which physically corresponds to a limit in the maximum stable static deflection. Second, the diaphragm dynamic motion is determined and the resulting sound pressure in the ear canal calculated. Using this method the linear response and distortion response may be separately analyzed. Both tend to increase in level as the transducer approaches the point of static instability. Experimental results agree well with the predictions of the theory. [Work done in part at Bell Laboratories, Murray Hill, NJ 07974.]

Foil electret transducer for blood pressure monitoring.

A foil electret microphone for use under the cuff of an automatic blood pressure monitoring system is described. The transducer is designed to operate with relatively flat sensitivity over a static pressure range of 40 to 250 mm Hg (5.33 X 10(4) to 3.33 X 10(5) dyn/cm2). The new electret microphone differs from conventional microphones used for airborne sound reception in two ways: (1) the diaphragm thickness is 50 micron rather than the typical 12.5 or 25 micron, and (2) the backplate contains a set of annular ridges spaced at 4 mm rather than the typical 7-10 mm. This microphone offers three advantages over the piezoelectric microphone now in use: (1) greater tolerance in positioning the microphone over the brachial artery, (2) nearly 20-dB higher sensitivity and signal-to-noise ratio, and (3) the ability to obtain measurements with the microphone placed midway between the elbow and shoulder. Tests of the new foil electret microphone in conjunction with the automatic blood pressure monitoring system indicate that the automatic and conventional measurements of systolic and diastolic blood pressure agree to within 5 mm Hg at least 90% of the time. In addition, the electret microphone is able to obtain automatic measurements on subjects with a wider range of ages and sizes.

A foil electret transducer for use under varying static pressure

We have built a foil electret microphone for use under‐the‐cuff in an automatic blood pressure monitoring system. The microphone uses a 50‐μ‐thick FEP diaphragm charged to − 250 V and suspended 35 μ above a back electrode. A flat sensitivity over a pressure range of 40 to 250 mm Hg (5.3 × 104 to 3.3 × 105 dyn/cm2) is required for this transducer. The sensitivity is maintained at a constant level by allowing the first contact between the electret and the back electrode to occur at 20 mm Hg. Although the microphone senistivity changes quickly from 0 to 20 mm Hg, it is relatively flat in the pressure range of interest. This design technique canbe used to provide a flat sensitivity for any foil electret microphone subjected to a varying static pressure.

Electret transducer backplate, electret transducer, and method of making an electret transducer

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New approach to directional foil electret transducers

The overall sensitivity of foil electret microphones has been shown to be independent of area [see G. M. Sessler and J. E. West, J. Acoust. Soc. Am. 40, 1433–1440 (1966)]. However, recent theoretical and experimental investigations have shown that within a single electret transducer, the sensitivity can be varied to affect directional response. Using sensitivity control, arrays of microphones can be replaced by a single transducer. In general, the single transducer is less affected by small errors in construction than its array counterpart.

Electret transducers for use in optoacoustic spectroscopy

The optoacoustic effect is based on the conversion of electromagnetic to sound energy, which is then detected by a microphone whose output is amplified. The resultant signal is predicted theoretically using a classical stretched drum type electret microphone as the transducer in a cylindrical optoacoustic cell filled with gas which absorbs single frequency laser radiation. These calculations simulate the conditions under which a series of experiments on chemical kinetics were performed; the experimental results are shown to be in good agreement with the predictions at gas pressures ≳ 0.5 Torr. Below 0.5 Torr, the microphone equation breaks down because the mean free path of the gas becomes comparable with the microphone electrode separation. This breakdown is observed experimentally at low gas pressure in the cell. Based on these considerations it is possible to use the optoacoustic effect to measure gas phase collisional relaxation rates and rate constants for homogeneous gas phase chemical reactions. Since some of the gases encountered are corrosive, gold is substituted for the usual aluminum evaporated onto the polymer to form the electret microphone diaphragm material.

Electret microphones with backplate electrets

Present electret transducers utilize either an electret diaphragm or an electret coated back electrode. Transducers based on the latter principle allow the freedom to use a variety of materials as the membrane. For example, proper membranes can be placed under high tension resulting in an increase in resonance frequency. Investigations of the average membrane deflection, and thus the sensitivity, of circular electret microphones show that the radiation impedance damps the resonance peaks and shifts them to lower frequencies. The phase shift between the incident sound wave and the ac output voltage is affected in a similar way. The influence of different surrounding gases on the frequency response can be calculated by changing the gas constants. Several gases, for instance helium, neon, CO2, and ethane, have been investigated. Studies of the vibration sensitivity of electret microphones, which is controlled by the mass of the membrane, the mass loading of the surrounding gas, and the sensitivity to airborne sound, are also presented.

Electret transducer having an electret of inorganic insulating material

An electret transducer in which the electret material is an inorganic insulator sheet with electric charges in it and is juxtaposed to an electrode, and either the sheet or the electrode is mechanically deformable to change the distance between the two, thereby changing the electric charge induced in the electrode by the charged electret sheet. In a preferred form, the electret material comprises a multilayer sheet of insulating materials. The method of forming the transducer includes at least the steps of vapor-depositing a thin layer of inorganic material on a substrate and subjecting the exposed surface of the insulating material to a source of electric charge for a time sufficient to implant charges in the insulating material. If the insulating material is to be deformable, it should be deposited on a thin layer of metal which, in turn, has been deposited on a thicker substrate. A spacer layer of metal is deposited on top of the charged insulating material. Thereafter, selected areas of the substrate and of the spacer layer are chemically removed to leave the thin metal layer and the charged insulating material free of support except around the periphery of such open area. The ability of the insulating material to accept and hold a charge may be improved by depositing the insulating material as two separate layers of different materials, one immediately on top of the other.