Electrical audio signal processing systems and devices – Electro-acoustic audio transducer – Having electrostatic element
Reexamination Certificate
2001-10-19
2003-06-24
Le, Huyen (Department: 2643)
Electrical audio signal processing systems and devices
Electro-acoustic audio transducer
Having electrostatic element
C381S174000, C381S402000
Reexamination Certificate
active
06584206
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a speaker apparatus, and more particularly to an electrostatic speaker apparatus in which desired frequency versus sound pressure and vibration amplitude characteristics can be achieved.
2. Description of the Related Art
As known in the art, a so-called electrostatic (capacitor-type) speaker includes a pair of fixed electrodes that face each other, and a vibrator disposed between the pair of fixed electrodes. Such an electrostatic speaker is designed so that a driving signal is applied across the pair of fixed electrodes and a d.c. bias voltage is applied between an electrode of the vibrator and each of the fixed electrodes, thereby generating sound pressure according to the driving signal.
FIGS. 8
to
10
illustrate the structure of a conventional electrostatic speaker
1
, by way of example. The electrostatic speaker (hereinafter referred to as “speaker apparatus”)
1
includes frames
1
a
and
1
b
which are screwed to each other. Inside the frames
1
a
and
1
b,
fixed electrodes
2
a
and
2
b
are placed so as to face each other.
The fixed electrodes
2
a
and
2
b
are exposed to the air at openings
3
a
and
3
b
formed in the center of the frames
1
a
and
1
b,
respectively. As shown in
FIG. 8
, the fixed electrodes
2
a
and
2
b
are made of a substantially rectangular planar plate, and have multiple openings formed therein.
In the following description with respect to the structure, generally, the fixed electrodes
2
a
and
2
b
and a diaphragm
4
formed therebetween are rectangular; however, the fixed electrodes
2
a
and
2
b
and the diaphragm
4
may have a circular or any other shape.
The diaphragm
4
is placed in a gap between the fixed electrodes
2
a
and
2
b.
The outer periphery of the diaphragm
4
is held between a metal frame
5
and a vibrator electrode
6
, and is fixedly received in the frames
1
a
and
1
b
through elastic members
7
.
The diaphragm
4
has air gaps each having a predetermined length formed with respect to the fixed electrodes
2
a
and
2
b.
Specifically, as shown in
FIG. 10
, spacers
8
a
and
8
b
are disposed in close proximity to the fixed electrodes
2
a
and
2
b,
respectively, so that the diaphragm
4
is held between the spacers
8
a
and
8
b.
In this structure, the thickness of the spacers
8
a
and
8
b
can be used to precisely set the lengths of the air gaps, namely, a distance da from the fixed electrode
2
a
to the diaphragm
4
and a distance db from the fixed electrode
2
b
to the diaphragm
4
.
Driving signals and d.c. bias voltages are applied to the speaker apparatus
1
having the above-described structure using a circuit shown in FIG.
11
.
The circuit includes a booster transformer T
1
and a transformer T
2
. A commercial power supply is connected to the primary winding of the booster transformer T
1
. The secondary winding of the booster transformer T
1
is connected to a multistage voltage doubling rectifier circuit including diodes D
1
to D
7
and capacitors C
1
to C
8
. The output of the multistage voltage doubling rectifier circuit is connected to the center tap of the secondary winding of the transformer T
2
.
The secondary winding of the transformer T
2
is connected to the fixed electrodes
2
a
and
2
b
through resistors R
3
and R
4
, respectively. One end of the secondary winding of the booster transformer T
2
is connected to the vibrator electrode
6
, or the diaphragm
4
, through a resistor R
1
and a terminal TC, so that d.c. bias voltages are applied between the diaphragm
4
and the fixed electrode
2
a,
and between the diaphragm
4
and the fixed electrode
2
b.
The d.c. bias voltages can be, for example, as high as 2.5 KV.
From a power amplifier connected to the speaker apparatus, audio signals are supplied across terminals which are connected to the primary winding of the transformer T
2
. As the audio signals are passed to the primary winding of the transformer T
2
via a resistor R
2
, they are boosted by the transformer T
2
before appearing on the secondary winding as the driving signals. The driving signals are thus applied to the fixed electrodes
2
a
and
2
b
through terminals TF and TR, respectively.
In the thus constructed electrostatic speaker
1
, driving forces F are expressed by the following general equation (1) of the Coulomb's law:
F
=
k
·
q
1
·
q
2
r
2
(
1
)
where q
1
and q
2
denote the charges of the electrodes, r denotes the distance between the electrodes, and k is the constant of proportion. Accordingly, the electrostatic speaker
1
is driven by the driving forces F calculated by general equation (1).
In the speaker apparatus
1
, the fixed electrodes
2
a
and
2
b
are substantially rectangular, and the electrode surfaces facing the diaphragm
4
are entirely parallel to the diaphragm
4
. In other words, the distance from any portion of the electrode surfaces to the diaphragm
4
is da or db shown in FIG.
10
. Therefore, the driving forces applied to the diaphragm
4
are uniform over the diaphragm surface.
Now, a case is considered where uniform driving forces F which are expressed by F=F
0
sin(&ohgr;t) are generated on the entire surface of the diaphragm
4
by the applied voltages. Then, the frequency characteristic of displacement distribution z across the diaphragm
4
is defined by the following equation (2):
z
=
∑
m
,
n
∫
(
F
0
Ξ
m
,
n
)
ⅆ
V
M
(
ω
m
,
n
2
-
ω
2
)
Ξ
m
,
n
sin
(
ω
t
)
(
2
)
where (m, n) indicates the order of the unique vibration mode of the diaphragm
4
, and &Xgr; denotes a reference function which indicates the displacement distribution in the unique vibration mode. Furthermore, V denotes the volume, M denotes the total weight of the diaphragm
4
, and &ohgr;
m,n
and &ohgr; denote the mnth-order resonance angular frequency and the angular frequency, respectively.
In the following description, the substantially rectangular diaphragm
4
has sides with lengths of a and b in the x- and y-axis directions, respectively, by way of example. The x-axis direction corresponds to the direction in which the longer sides extend, and the y-axis direction corresponds to the direction in which the shorter sides extend, as viewed from the surface direction of the diaphragm
4
, as shown in FIG.
8
. In this case, the displacement distribution in the unique vibration mode is given by the following equation (3):
Ξ
m
,
n
=
sin
m
π
a
x
·
sin
n
π
b
y
(
3
)
When equation (3) is substituted into equation (2), the following equation (4) is derived with respect to the displacement distribution z across the diaphragm
4
:
z
=
∑
m
=
1
,
3
,
5
…
n
=
1
,
3
,
5
…
4
F
0
mn
π
2
abM
(
ω
m
,
n
2
-
ω
2
)
Ξ
m
,
n
sin
(
ω
t
)
(
4
)
FIGS. 7A
to
7
D show four resonance modes as portions of the vibration modes indicated by equation (4).
FIG. 7A
shows a resonance mode where (m, n)=(1, 1) in equation (4), showing the state where the entire surface of the diaphragm
4
vibrates in the same direction at a particular frequency. In a resonance mode where (m, n)=(3, 1) at a higher frequency, as shown in
FIG. 7B
, a vibration is created in a ridge-trough-ridge fashion along the longer sides (in the x-axis direction).
FIG. 7C
shows a resonance mode where (m, n)=(1, 3) in which a vibration is created in a ridge-trough-ridge fashion along the shorter sides (in the y-axis direction).
FIG. 7D
shows a resonance mode where (m, n)=(3, 3) in which a vibration is created in a composite ridge-trough fashion along the longer sides and the shorter sides.
Therefore, the diaphragm
4
has a characteristic that multiple resonance modes may be generated, and the frequency characteris
Le Huyen
Maioli Jay H.
Sony Corporation
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