GB2124058A - A counter electrode for an electrostatic transducer - Google Patents

Abstract

To reduce the stray capacitance of a counter electrode for an electrostatic transducer e.g. a capcitor or electret microphone the counter electrode (15) is of wafer form and is formed with recesses (17) at its periphery. The area of the counter electrode bounded by the recesses may have the form of a circle, an ellipse, an oval or a regular polygon, such as a square, rectangle or hexagon. <IMAGE>

Description

SPECIFICATION A counter electrode for an electrostatic transducer The invention relates to a counter electrode for an electrostatic transducer, more particularly a capacitor microphone.

West German Auslegeschrift 1437420 and West German Offenlegungsschrift both describe a capacitor microphone having a counter electrode which is situated at a distance of 301.L m to SOIL m behind the diaphragm and forms a rigid immovable electrodes. The counter electrode is usually in the form of a flat electrically conductive disc. The disc is formed with a relatively large number of boreholes or slots in order either to couple a very shallow and small air chamber between the diaphragm and the counter electrode to other acoustically active means inside the microphone casing or else, as is necessary for directional microphones, to connect the air chamber to the external sound field via coupled acoustic means.

In electrostatic transducers, more particularly in capacitor microphones, the capacitance is determined substantially by the area of the counter electrode, the area of the electrically conductive diaphragm situated in front of the back plate, and the distance therebetween. When sound impinges on the capacitor microphone diaphragm, only part of the total capacitance is usually active for signal transmission. The balance of the total capacitance, that is, the stray capacitance, which is caused mainly by stationary parts necessary to the design, results in a deterioration in the transmission factor, the signal-tonoise ratio and the non-linear distortion. It has therefore proved necessary to make the usable capacitance/stray capacitance ratio as large as possible.

With a diameter in excess of 1 5 mm previous counter electrodes have enabled the usable capacitance/stray capacitance ratio to be made sufficiently large to keep negative effects, that is deterioration of the transmission factor, signal-to-noise ratio and non-linear distortion, within tolerable limits.

However, the increases in miniaturization which require a diaphragm, and counter electrode, diameter of less than 7 mm means that the stray capacitance becomes unacceptably large with respect to the usable effective capcitance. For example, with an electrode diameter of 5.8 mm and a distance of 40 ,um between the diaphragm and the counter electrode with a previously proposed counter electrode construction, the stray capacitance will be, for example, 3.4 pF (pico-farads) while the effective capacitance will be 3 pF.

According to one aspect of the present invention, there is provided a counter electrode for an electostatic transducer wherein the counter electrode is formed with recesses at the periphery thereof.

Preferably, the counter electrode is in the form of a wafer.

The area of the counter electrode bounded by the recess may have, for example, substantially the form of a circle, an oval, an ellipse or a regular polygon such as a square, a rectangle or a hexagon.

The counter electrode is preferably made from metal, metal-coated plastics material, electrically conductive plastics material, electretizable material or from metal to which an electretizable material has been applied.

The invention also provides an electrostatic transducer incorporating a counter electrode in accordance with the first aspect.

According to a second aspect of the present invention, there is provided an electrostatic transducer comprising a diaphragm and counter electrode, the counter electrode being in the form of a wafer and being formed with recesses at the periphery thereof.

Generally, the transducer is a capacitor microphone.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompany ing ing drawings, in which: Figures la to ic are plan views of various previous counter electrode constructions; Figure 2 is a cross-section of a capacitor microphone having a counter electrode embodying the invention; Figure 3 is a plan view of a circular counter electrode embodying the invention; and Figures 4a to 4d are plan views of various different shapes of counter electrodes embodying the invention.

Referring now to the drawings, Figs. 1 a to 1 c show plan views of previously proposed counter electrodes 1, 2, 3 for capacitor microphones. The counter electrodes 1, 2, 3 are formed with apertures, in particular drilled apertures. In the arrangement shown in Figs.

1 and 2, the apertures are in the form of boreholes 4 generally arranged in a geometric pattern whereas in the arrangement shown in Fig. 3, the apertures take the form of slots 5 arranged in the shape of a circle. Other acoustic means disposed behind the counter electrode can be coupled to the shallow air chamber between the diaphragm and the counter electrode of the microphone both via the bores and via the slots. The area in the region of the edge or periphery of the counter electrode which contributes to the formation of stray capacitance is shown as a dotted zone 6.

The capacitor formed by the circular ring produced by the annular zone 6 and the opposite diaphragm, which acts as the electrode, has a capacitance greater than half the total capacitance produced by the diaphragm and the counter electrode.

Fig. 2 is a cross-section of a capacitor microphone having the counter electrode embodying the invention. A diaphragm 9 of the microphone secured to a diaphragm retaining ring 8, together with an electrically insulating spacer ring 10 and a counter electrode 11, is pressed against a shoulder 7b of a casing 7 of the microphone by a retaining spring 1 2 disposed inside the microphone casing 7. The casing 7 is formed with sound inlet apertures 7a at the front thereof and the retaining spring 1 2 bears against a baseplate 1 3. The counter electrode 11, which is in the form of a wafer, is formed with recesses 11 a along its periphery, the recesses enabling the air chamber 14 to be coupled via the slots 1 4a to acoustic means behind the counter electrode 11.

Figure shows a preferred shape of a counter electrode 1 5 embodying the invention. The counter electrode 1 5 is circular and is provided with star-shaped projections 1 6 formed by recesses 1 7 at the periphery of the counter electrode. The counter electrode areas involved in forming the stray capacitance are shown as dotted zones 18 on the projections 1 6. These zones are so small compared with the total counter electrode area that the stray capacitance is only one-third of the total capacitance, thus giving a favourable "effective capacitance"/"stray capacitance" ' ratio.

Figs. 4a to 4d show other counter electrodes embodying the invention. The contours 19 and 20 of the counter electrodes shown in Figs. 4a and 4b bounded by recesses 1 9a and 20a, respectively, form a polygon while the counter electrode shown in Fig. 4c has an oval contour 21 bounded by recesses 21 a and the counter electrode shown in Fig. 4d has elliptical contour 22 bounded by recesses 22a.

As a result of the star-like webs left at its periphery, a counter electrode embodying the invention has a stray capacitance of 1.3 pF compared with a usable effective capacitance of 3 pF for a total counter electrode diameter of 5.8 mm. The usable capacitance/stray capacitance ratio is thus restored to a favourable value. More particularly, the sensitivity of the capacitor microphone is increased by almost 30%, compared to that of conventional capacitor microphones, for total diameters less than 7 mm.

The recesses 11 a, 1 9a, 20a, 21 a or 22a at the periphery of the counter electrode form a passage to the rear for air present in the shallow and small air chamber 14 between the diaphragm and the counter electrode. The function of the boreholes or slots in a conventional counter electrode is performed by the recesses in cooperation with the spacer ring between the counter electrode and the diaphragm, so that coupling of other acoustically active means, for example, acoustic friction, spring means or masses, and the connection to the sound field necessary for directional microphones, is by way of the slots formed by the spacer ring and the recesses.

The manufacture of a counter electrode in the form of a wafer embodying the invention, particularly a counter electrode of star shape, can be carried out very simply and advantageously, by stamping out the material. When the counter electrode diameter is less than 7 mm, considerable difficulties occur with the previous counter electrodes in defining a suitable diameter for the holes which are to be punched out, in relationship to the thickness of the counter electrode material. It is frequently necessary to resort to etching methods, which are more expensive than punching. A counter electrode embodying the invention eliminates the very difficult operation of punching holes or slots in plastics such as, for example, Teflon (Registered Trade Mark), for electretizable counter electrodes, because the recesses formed at the periphery of the wafer are easily stamped out.

Because the stray capcitance is operative only in the region of the edge of the counter electrode, the recessed area is advantageously made so large as to reduce the stray capacitance as far as possible. Preferably, the recessed area is equivalent to up to 70% of the remaining counter electrode area. Consequently, the usable capacitance/stray capacitance ratio actually assumes the value required to obtain a high-grade capacitor microphone in respect of high sensitivity, good transmission factor, good signal-to-noise ratio and low non-linear distortion.

As shown in Figs. 4a to 4d, the counter electrode area defined by the recesses need not necessarily be in the form of a circle but, if required, can take the form of a polygon, an oval or an ellipse. For practical reasons, however, the circular form will be generally used in the production of capcitor microphone.

Other forms are advantageous, however, for special applications, for example, hearing aids for the hard of hearing, so that, generally speaking a polygonal, oval or an ellipical shape are feasible as peripheral boundaries for the counter electrode, and hence also for the diaphragm, of a condenser microphone.

A counter electrode embodying the invention may, whether the bias voltage is externally supplied to the transducer or produced therein by means of an electret, be formed of metal, metal-coated plastics or a material acting as an electret.

Thus, using a counter electrode embodying the present invention the usable effective capacitance may be up to about 70% of the total capacitance while the stray capacitance is about 30% of the total capacitance.

Claims (14)

1. A counter electrode for an electrostatic transducer wherein the counter electrode is formed with recesses at the periphery thereof.

2. A counter electrode according to claim 1, wherein the counter electrode is in the form of a wafer.

3. A counter electrode according to claim 1 or 2, wherein the recessed area is equivalent to up to 70% of the remaining area of the counter electrode.

4. A counter electrode according to claim 1, 2 or 3, wherein the area of the counter electrode bounded by the recesses is substantially in the form of a circle, an oval, an ellipse.

5. A counter electrode according to claim 1, 2 or 3, wherein the area of the counter electrode bounded by the recesses is a regular polygon.

6. A counter electrode according to claim 5, wherein the regular polygon is a square, rectangle or hexagon.

7. A counter electrode according to any preceding claim wherein the counter electrode is formed of metal, a metal-coated plastics material, an electrically conductive plastics material, an electretizable material, or of metal having an electretizable material applied thereto.

8. A counter electrode substantially as hereinbefore described with reference to, and as illustrated in, Fig. 3, 4a, 4b, 4c or 4d of the accompanying drawing.

9. An electrostatic transducer having a counter electrode in accordance with any preceding claim.

10. An electrostatic transducer comprising a diaphragm and counter electrode, the counter electrode being in the form of a wafer and being formed with recesses as the periphery thereof.

11. An electrostatic transducer according to claim 9 or 10, wherein the transducer is a capacitor microphone.

1 2. An electrostatic transducer substantially as herein before described with reference to, and as illustrated in Figs. 2 and 3 of the accompanying drawing.

1 3. An electrostatic transducer substantially as hereinbefore described with reference to, and as illustrated in, Fig. 2 of the accompanying drawing when modified by Fig. 4a, 4b, 4c or 4d of the accompanying drawing.

14. Any novel feature or combination of features described herein.