Electrical audio signal processing systems and devices – Electro-acoustic audio transducer – Microphone capsule only
Reexamination Certificate
2001-06-28
2003-02-25
Le, Huyen (Department: 2643)
Electrical audio signal processing systems and devices
Electro-acoustic audio transducer
Microphone capsule only
C381S191000
Reexamination Certificate
active
06526149
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a system and method for reducing non-linear electrical distortion in an electroacoustic device. Specifically, the present invention relates to a system and method for reducing spatially dependent electrical distortion, for example, distortion caused by differences in electrical displacement between a conductive membrane and a counter electrode at different parts of the conductive membrane. The system and method for reducing non-linear electrical distortion has particular application in condenser microphones comprising, for example, a conductive diaphragm receptive to sound, and a backplate electrically coupled thereto to generate an electrical output.
2. Description of Related Art
Generally, an electroacoustic device such as, for example, a condenser microphone, converts sound pressure input into electrical output.
FIG. 1
illustrates an example of a condenser microphone system
10
. The microphone
10
comprises a conductive membrane
12
exposed to sound
24
, electrically coupled to a counter electrode
14
. A conductive membrane
12
may be, for example, a diaphragm. Examples of counter electrodes include a back electrode, a backplate, and a conductive front grille. Generally, the counter electrode
14
is stationary and displaced in parallel and in close proximity to the conductive membrane
12
, such that the combination of the electrode
14
and membrane
12
acts as a capacitor capable of storing a charge. A polarizing voltage, Eo, is applied either through a polarizing voltage source
16
and polarization resistor
18
, or by using an electric layer (not shown), by which a constant initial charge is established. This constant initial charge provides initial voltage when the diaphragm
12
is not affected by sound waves
24
, that is, when the diaphragm
12
is in a rest position. Additionally, the exemplary condenser microphone
10
may include one or more parasitic parallel load capacitors
20
and a preamplifier section
22
to produce an amplified electrical output
26
.
When sound impinges on the membrane
12
, it moves, thus changing capacitance between itself and the counter electrode. This produces a variable voltage, which is the electrical output signal, E. In an ideal electroacoustic system, the electrical output, E, varies linearly with the pressure of actuating sound waves
24
upon the membrane
12
. A linear movement/output voltage relationship means that the diaphragm
12
maintains a parallel relationship with the counter electrode
14
. This assumes, in the ideal case, that displacement due to a given sound pressure is in a direction perpendicular to the parallel relationship and equal in magnitude at all areas of the diaphragm
12
.
FIG. 2
illustrates the parallel displacement, d, of the diaphragm
28
in relation to counter electrode
30
in an ideal system
27
. Assuming, for simplicity, that no parasitic parallel load capacitances exist,
C
a
=
C
0
/
(
1
+
d
/
D
)
,
and
E
=
E
0
C
0
C
a
=
E
0
(
1
+
d
/
D
)
,
where
C
a
is the active capacitance (varies with diaphragm displacement),
C
0
is the capacitance at rest position,
d is the displacement of membrane with sound pressure,
D is the rest distance between counter electrode and diaphragm,
E is the output voltage (constitutes signal), and
E
0
is the polarization voltage (voltage at rest position).
Therefore, in an ideal system, the output voltage varies linearly with the displacement between diaphragm
28
and counter electrode
30
. However, in a typical electroacoustic system, the relationship between electrical output, E, and sound pressure is non linear.
FIG. 3
illustrates a typical system
31
, comprising a flexible diaphragm
32
, stretched and clamped at its edges. The particular tension of the diaphragm and the method of attaching are well understood in the art, and are not important for an understanding of the present invention. The displacement of the diaphragm
32
to sound at frequencies below resonance is parabolic, such that an equal distance between the diaphragm
32
and counter electrode
34
is not maintained from all areas of the diaphragm
32
. For example, D/d(r
1
) is not equal to D/d(r
2
). Therefore, the change of capacitance between diaphragm
32
and counter electrode
34
is spatially dependent. This spatial dependence manifests itself as a signal dependant stray capacitance which makes active capacitance, C
a
, nonlinear. It has been shown that the displacement of such a diaphragm may be represented by d(r)=d
0
(1−r
2
/R
d
2
), where
r—radial coordinate with its origin at the center of the diaphragm,
R
d
—radius of the diaphragm
d
0
—diaphragm displacement at the center.
The initial capacitance between the diaphragm
32
and counter electrode
34
, that is, the capacitance without displacement of the diaphragm
32
, may be represented by,
C
0
=
ϵ
·
π
·
R
b
2
D
.
Then, the active capacitance, C
a
, and output voltage, E, may be represented by
C
a
=
∫
0
R
b
ϵ
·
2
π
·
r
D
+
d
0
(
1
-
r
2
/
R
d
2
)
ⅆ
r
=
ϵ
·
2
π
·
R
b
2
D
·
R
b
2
∫
0
R
b
2
r
1
+
d
0
D
(
1
-
r
2
/
R
d
2
)
ⅆ
r
=
C
0
·
R
d
2
R
b
2
·
D
d
0
·
ln
1
+
d
0
/
D
1
+
(
1
-
R
b
2
/
R
d
2
)
·
d
0
/
D
,
and
E
=
E
0
C
0
C
a
=
E
0
·
R
b
2
R
d
2
·
d
0
D
·
1
ln
1
+
d
0
/
D
1
+
(
1
-
R
b
2
/
R
d
2
)
·
d
0
/
D
,
where
R
b
is the radius of the counter electrode.
As the equation for E shows, the relationship between the output voltage E and diaphragm displacement is non-linear.
FIG. 4
provides a graph
36
illustrating the non-linearity of the relationship between the output voltage and diaphragm displacement in a typical system
38
as compared to the linear relationship in an ideal system
40
. The problem lies in the fact that the ratio of the motion to the distance to the counter electrode is different for different areas of the diaphragm
32
.
SUMMARY OF THE INVENTION
The present invention provides a system and method for reducing variations in electrical displacement between different sections of a conductive membrane and a counter electrode electrically coupled to the conductive membrane in an electroacoustic device.
One embodiment of the present invention provides a system comprising a counter electrode, a face of which faces the conductive membrane and is curved to minimize the variations in electrical displacement between different sections of the conductive membrane and the counter electrode.
In another embodiment, the counter electrode is variably polarized to counteract for variations in displacement across the conductive membrane. In yet another embodiment, the diaphragm is variably polarized for this purpose.
In an alternative embodiment, the present invention provides a system comprising a translational apparatus for translating a rigid conductive membrane laterally to maintain its parallel relationship to the counter electrode during reverberations.
It will be appreciated by those skilled in the art that although the following Detailed Description will proceed with reference being made to exemplary embodiments and methods of use, the present invention is not intended to be limited to these exemplary embodiments and methods of use. Rather, the present invention is intended to be limited only as set forth in the accompanying claims.
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patent: 4620446 (1986-11-01), Jensen et al.
patent: 4694499 (1987-09-01), Bartlett
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patent: WO 00/35247 (2000-06-01
Blackmer David E.
Khenkin Aleksey S.
Earthworks, Inc.
Grossman Tucker Perreault & Pfleger PLLC
Le Huyen
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