Electret Transducer

Abstract

An electret transducer comprising a metallized diaphragm tensioned in juxtaposition with a metallic backplate in which the width to height of the air gap between the two is selected so that the sensitivity remains substantially constant over the life of the transducer. The backplate contains ridges over which the diaphram is tensioned.

Description

FIELD OF THE INVENTION

This invention relates to an electret transducer and more particularly to one in which the dimensional parameters are selected so that the sensitivity remains substantially constant as the surface charge density of the electret decays.

DESCRIPTION OF THE PRIOR ART

Recently, there has been considerable development of electret transducers which may be utilized as either microphones or loudspeakers. One common form utilizes a metallized thin plastic film or diaphragm which is tensioned in close proximity to a conductive backplate. The film has a permanent charge on it so that when an electrical signal is connected across the two conductive surfaces the diaphragm is alternatively attracted to and repelled from the backplate so as to vibrate in the manner of a loudspeaker.

Conversely, when sound pressure waves are applied to the diaphragm an electrical signal is produced between the two conductive surfaces in a manner similar to that of a condenser microphone. However, unlike the condenser microphone, no external d-c bias is required. Because the bias in the condenser microphone is supplied from an external source, it can be readily maintained within close limits. Hence, the sensitivity can be held within strict limits as a result of which the condenser microphone is often used as a standard in sound measurements. However, in the electret microphone, the initial surface charge density on the electret film slowly decays over the useful life of the transducer. This is particularly true when the transducer is subjected to high humidity and temperature, both of which accelerate the charge decay process.

SUMMARY OF THE INVENTION

It has bee discovered that, in an electret transducer in which the diaphragm is spaced from the backplate by a number of parallel ridges, by selecting a width between ridges to ridge height ratio within a certain range, the sensitivity remains substantially constant over a wide range of surface charge densities. As a result, the sensitivity of the microphone can be held within close tolerances over its useful life.

Thus, in accordance with the present invention there is provided an electret transducer, which may be either in the form of a microphone or a loudspeaker, comprising a perforated backplate having both a conductive surface and a plurality of parallel ridges on one side. In addition, the transducer includes an electret film or diaphragm tensioned over the parallel ridges with a metallized surface on the side opposite that in contact with the ridges. The ratio of the distance between the ridges to the height of the ridges is between about 100 to 1 and 40 to 1 as a result of which the sensitivity remains substantially constant as the surface charge density on the electret diaphragm decays over its useful life.

The tension on the diaphragm controls the resonant frequency of the transducer. Thus, a higher tension will produce a higher resonant frequency. However, a lower tension will produce a greater sensitivity up to the point where the diaphragm collapses. This occurs when a decrease of the air gap, which may be due to an applied positive pressure, causes the electrostatic force due to the charge to increase by an amount greater than the elastic restoring force that is determined by the tension on the diaphragm. In "Electroacoustics" pp. 179-183, 1954, Harvard University Press, Harvard Monograph in Applied Science Number 5, F.V. Hunt has shown that a theoretical limit of stability is reached when the static air gap, due to a field in the gap, is reduced by one-third that when no field is applied. Thus the electrostatic force required for collapse will depend upon the tension on the diaphragm as well as other parameters which affect its compliance. In a preferred embodiment, the tension is between about 10 Newtons per meter and 20 Newtons per meter which provides maximum sensitivity while satisfying the other parameters mentioned above.

The sensitivity of the transducer is dependent upon the amount of fixed capacity in shunt with the variable capacity. For maximum sensitivity, the former should be kept as small as possible. The backplate may be constructed of a plastic material on which is deposited a metallized coating. In order to reduce the fixed capacity of the transducer in a still more preferred embodiment, only a small portion of each of the ridges is metallized in order to interconnect the flat surface portions of the backplate.

It has been found that utilizing a metallized plastic backplate yields an additional advantage in that the coefficient of expansion of the backplate can be selected to be substantially the same as that of the diaphragm. This maintains a substantially constant tension on the film with variations in ambient temperature. It has been achieved in the present invention by utilizing metallized acrylonitrile butadiene styrene for the backplate and metallized tetrafluoroethylene for the diaphragm.

BRIEF DESCRIPTION OF THE DRAWINGS

An example embodiment of the invention will now be described with reference to the accompanying drawings in which:

FIG. 1 is an exploded view of an electret transducer in accordance with the present invention;

FIG. 2 is a cross-sectional view of the assembled electret transducer shown in FIG. 1; and

FIG. 3 is a graph of signal output versus equivalent bias voltage for the electret transducer illustrated in FIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 1 and 2, the electret transducer shown in the form of a microphone comprises a slightly convex backplate 10 having a plurality of parallel ridges 11 on one side thereof. In the present embodiment, the backplate 10 was constructed of plastic. In order to provide a conductive surface, portions 12 of the backplate 10 were covered with a metallized coating. Overlying the periphery of the backplate 10 is a thin plastic gasket 13 which helps to reduce the stray fixed capacity of the microphone. Stretched over the gasket 13 in contact with the ridges 11 is a diaphragm comprising a thin plastic film 14 having a metallized coating 15 on the side opposite that in contact with the ridges. A frame 16 is placed over the diaphragm 14. The whole assembly is held together by a pair of clips 17 and 18 which are snapped over opposed ends of the assembly.

To relieve the back pressure between the diaphragm 14 and the backplate 10, a series of holes 19 are located in the backplate 10. It has been found that holes of 40 mils diameter on 80 mil centers achieve this without excessively reducing the effective area of the conductive portions 12 and hence the variable capacity of the transducer.

An external connection is made to the conductive portions 12 of the backplate 10 through an electrical connecting portion 20. Electrical connection to the metallized coating 15 is made by extending the diaphragm 14 around and under the bottom of the clip 18 as shown on the right side of FIG. 2. Hence, the metallized coating 15 contacts the clip 18, to which suitable connection can then be made.

While various materials may be used, it has been found that good results are obtained if the electret film 14 is made of tetrafluoroethylene which is sold under the trademark Teflon. The thickness of the film 14 will also affect the performance of the transducer. It has been found that very good results can be obtained using 1/2 mil Teflon. The thickness of the metallized coating 15 on the Teflon film 14 is only a small fraction of the total thickness of the film 14, typically in the order of 1,000 to 5,000 angstroms. The electret film 14 may be charged using any one of a number of known charging methods. One such method is disclosed in "Thermal Currents from Corona Charged Mylar" by Robert A. Creswell and Martin M. Perlman; Journal of Applied Physics, Vol. 41, No. 6, pages 2365-2375, May 1970. It has been found that the charge on the electret film 14 has a longer life if the uncoated side of the film 14 (that facing the backplate 10) is charged negatively.

While the Teflon film 14 holds a permanent charge very well even in areas of high humidity, it suffers from the disadvantage that when subject to a tension, it tends to relax or stretch over a prolonged period, particularly with expansion and contraction due to variations in ambient temperature. This tendency can be reduced by selecting a material such as acrylonitrile butadiene styrene for the backplate 10 and the frame 16 having similar coefficient of expansion characteristics to that of Teflon. The use of a plastic for the backplate 10 has the further advantage in that only the selected portions 12 contiguous with the movable areas of the film 14 need be metallized as shown in FIG. 1. This helps to greatly reduce the stray fixed capacity of the transducer. Since the overall sensitivity of the transducer is dependent upon the ratio of the movable to fixed capacity of the transducer, the result is increased sensitivity. The stray capacity can be further reduced as shown in FIG. 1 by metallizing only a small portion 21 of the ridges interconnecting the various metallized sections 12.

The sensitivity remains substantially constant, i.e. within a few decibels, as the surface charge density on the electret decays over its useful life by selecting a width W to height H ratio of between about 40 to 1 and 100 to 1, as shown in FIG. 2. However, optimum results appear to be obtained if the ratio of W to H is in the range of 70 to 1 with a ridge height H of about 2 mils, while the transducer is working into a typical load (not shown) such as 6 M.OMEGA. shunted by 30 pF.

The surface charge density is usually measured by determining the equivalent bias voltage required to neutralize the electrostatic field across the air gap between the plastic film 14 and the backplate portions 12. In a microphone this is measured by connecting a variable d-c source and a load (both not shown) in series with the microphone. An acoustic signal is then fed to the microphone and the bias is varied until the resultant a-c signal across the load is nulled. At this point, the d-c voltage from this bias source is read as the equivalent bias voltage of the surface charge density.

The relationship between the two is given as follows:

.sigma. = K.epsilon..sub.o V.sub.b /d

where:

.sigma. = surface charge density;

K = dielectric constant;

.epsilon..sub.o = permittivity of free space;

V.sub.b = equivalent bias voltage; and

d = thickness of electret film 14.

A graph of signal output versus equivalent d-c bias for various tensions on the film 14 is shown in FIG. 3 for ratios of W versus H (FIG. 2) of 70:1. Evidently, the sensitivity remains substantially flat over the equivalent bias voltage range of from 50 to 200 volts.

It should be noted that the sensitivity may remain substantially constant for equivalent bias voltages greater than 200 volts. However, this is considered to be a practical limit since it is not only difficult to charge the film 14 to an equivalent bias voltage greater than 200 volts but there is also an increased tendency towards arcing across the film 14 itself for values beyond this point.

Evidently, the sensitivity is inversely proportioned to the tension on the electret film 14 while the resonant frequency is nearly proportional to the square root of the tension. In the preferred embodiment the film is tensioned in a direction orthogonal to that of the length of the ridges. The tension is held between about 10 and 20 Newtons per meter. This yields good sensitivity and reproductibility yet leaves sufficient safety margin that the film 14 is not in danger of collapsing.

Claims

What is claimed is:

1. An electret transducer comprising:

a perforated backplate having both a conductive surface and a plurality of parallel ridges on one side thereof; and

an electret diaphragm tensioned over the parallel ridges; the diaphragm having a metallized surface on the side opposite that in contact with the ridges;

in which the ratio of the distance between said ridges to the height of the ridges is between about 100:1 and 40:1 whereby the sensitivity of the transducer remains substantially constant when the equivalent bias voltage resulting from the surface charge density of the electret varies between about 200 and 50 volts.

2. An electret transducer as defined in claim 1 in which the tension on the diaphragm is between about 10 and 20 Newtons per meter.

3. An electret transducer as defined in claim 2 in which the ratio of the distance between said ridges to the height of the ridges is about 70:1.

4. An electret transducer as defined in claim 1 which has a plurality of conductive areas between said ridges interconnected by narrow conductive portions over the ridges so as to minimize the fixed capacity of the transducer.

5. An electret transducer as defined in claim 1 in which the coefficient of expansion of the perforated backplate and the electret diaphragm are substantially the same.

6. An electret transducer as defined in claim 5 in which the material in the backplate consists of acrylonitrile butadiene styrene and the material in the diaphragm consists of tetrafluoroethylene.

7. In an electret transducer comprising:

a perforated backplate having a plurality of parallel ridges on one side thereof;

an electret diaphragm tensioned over the parallel ridges, the diaphragm having a metallized surface on the side opposite that in contact with the ridges;

the improvement comprising:

said backplate having conductive areas between said ridges interconnected by narrow conductive portions over the ridges so as to minimize the fixed capacity of the transducer.