U.S. patent number 3,663,768 [Application Number 05/106,776] was granted by the patent office on 1972-05-16 for electret transducer.
This patent grant is currently assigned to Northern Electric Company Limited. Invention is credited to Henning Schmidt Madsen, Cornelis Wilfred Reedyk.
United States Patent |
3,663,768 |
Madsen , et al. |
May 16, 1972 |
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.
Inventors: |
Madsen; Henning Schmidt
(Hillerod, DK), Reedyk; Cornelis Wilfred (Ottawa,
Ontario, CA) |
Assignee: |
Northern Electric Company
Limited (Montreal, Quebec, CA)
|
Family
ID: |
22313174 |
Appl.
No.: |
05/106,776 |
Filed: |
January 15, 1971 |
Current U.S.
Class: |
381/191; 381/427;
381/174 |
Current CPC
Class: |
H04R
19/01 (20130101) |
Current International
Class: |
H04R
19/00 (20060101); H04R 19/01 (20060101); H04r
019/00 () |
Field of
Search: |
;179/111E ;307/88ET |
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: Olms; Douglas W.
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.
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.
* * * * *