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BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
condenser microphone, and more particularly to the improvement of a diaphragm that vibrates
due to sound pressure. 2. Description of the Related Art In condenser microphones, particularly
nondirectional condenser microphones, generally, large aperture units are more likely to disturb
high frequency characteristics than small aperture units. One of the factors is that the tension of
the polymer film such as polyester constituting the diaphragm is limited, and the high frequency
reproduction limit is low. Therefore, the diameter of the diaphragm is limited to such an extent
that the high frequency characteristic is not disturbed. SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a condenser microphone in which the
high frequency characteristics are not disturbed even if the diameter of the diaphragm is
increased. Another object of the present invention is to provide a condenser microphone capable
of extending the life of the electret in a condenser microphone having an electret between a
diaphragm and a back electrode. [Configuration of the invention] That is, according to the
present invention, a plurality of spacers are provided on the surface of the diaphragm opposite to
the back electrode at an appropriate distance, and the surfaces of each spacer are in contact with
the back electrode. It is characterized in that it is provided in contact. The invention is also
characterized in that the above-mentioned spacer is constituted by an electret. The present
invention will now be described in detail with reference to an embodiment shown in the
accompanying drawings. As shown in FIG. 1, this condenser microphone 1 is provided with a
cylindrical casing 2 made of aluminum or the like having a plurality of through holes 3 on the
sound receiving surface. In the casing 2, the diaphragm 4 is disposed in a state of being attached
to the support ring 5, and the back electrode 6 is opposed so as to form a predetermined air gap
between the diaphragm 4 and the diaphragm 4. It is arranged. In this case, the back electrode 6
is placed on a support 8 made of an electrically insulating material through a spacer ring 7
having conductivity, and an acoustic wave passage is provided in each of the back electrode 6
and the support 8. A plurality of through holes 9 and 10 forming the hole are formed
respectively. In this embodiment, the back electrode 6 is, for example, a field effect transistor
(not shown) which constitutes a preamplifier via the spacer ring 7 and the external thread 11
fixing the spacer ring 7 to the support 8. It is connected to the gate, while the diaphragm 4 is
connected via its support ring 5 and the casing 2 to the drain of the field effect transistor.
On the diaphragm 4, as shown in FIG. 2, on the diaphragm 4 in the six-ring spacer 12 with the
ring spacer 12 attached at its periphery, a plurality of disk-shaped disks are provided at
appropriate intervals. Spacer 13 is attached. These spacers 13 may be formed of
electrets, whereby a polarization voltage can be obtained and the clearance between the
diaphragm 4 and the back electrode 6 can be maintained. By providing a large number of pointlike or small planar support points on the diaphragm 4 by the spacer 13 in this manner, the
diaphragm 4 substantially becomes a form in which a plurality of small diameter diaphragms 4
are connected in parallel. The frequency characteristics of can be improved as compared to those
without support points. The spacer 12.13 may be attached onto the diaphragm 4 by means such
as printing or heat sealing other than adhesion. Here, in order to compare the conventional
condenser microphone and the condenser microphone of the present invention, experimental
data of measurement of frequency characteristics of a nondirectional condenser microphone unit
having a diameter of 16 m is shown in FIG. The conventional condenser microphone is a flat
diaphragm. In the present invention, a spacer dot of 0.4 m in diameter is spaced by 2 rNa with
epoxy ink on the diaphragm, and 606 dots are each for the spacer dot that is in contact It is
provided to be positioned at an angle. As apparent from the characteristic diagram of FIG. 3,
although the conventional unit shows disturbance in the frequency characteristic from 5 to L oku
Hz, in the present invention, the above-mentioned frequency band is substantially flat and the
characteristic disturbance is You will see that it is small. Further, the diameter of the diaphragm
4 is d1 = m, the inner diameter of the support ring 5 is d, + =. Phi.10.times.10 ""
m, and the diameter of the disk-like spacer 13 consisting of FEP is dd = .phi.0. 8 × 10− ′ ′,
and the thickness is dG = 25 × 10− @ m, and the diameter of the through hole 9 of the back
electrode 6 is dP = φ0.5 × IO−′m. The number of spacers 13 is N 1 = 20, and the number of
through holes 9 is N 2 = 10. Here, the area S1 of the diaphragm 4 is 1.77 × 10−a, and the area
S2 of the diaphragm exposed in the support ring 5 is 7.8 × 10−′rr? The total area S3N1 of the
spacer 13 is 1.0XIO-'rrr, and the total area 84N2 of the through hole 9 of the back electrode 6 is
i, 9xto'rrf.
Therefore, the electrostatic capacitance C of the capacitor portion is given by the following
equation. C = ((St-82) k + (St-3 11 Nt-84 N 2) + S = I N 1 k-where the dielectric constant of the
electret material FEP is 2.0 at 1 kHz. Is the dielectric constant of vacuum, and is 8.85 × 10 6 F /
m. By applying the above-mentioned values to this equation, C = 100 PF 6 Therefore, the surface
charge density is 1. Assuming that X10 ′ ′ ′ C / port, the total charge amount Q is 1.09 ×
10− ′ ′ (C), and a voltage of V = 109 V (7) is given. The effective vibration area of the
diaphragm is 6.8 × to ′ ′ (rrF) and can be sufficiently used. In addition, a plurality of spacers
with a diameter of 0.8XIO-3m and a thickness of 25X10- "m are provided on the diaphragm 4 of
metal foil having a diameter of 10XIO-'m and a thickness of 25 x 10- @ m, and 60V polarization
is provided. When considering the case of obtaining a voltage, the capacitance CN3 of the
capacitor section is the number of spacers, and k and to have the same coefficients as the above
equation (1). By the electretization, the charge sensitivity held by the spacer FEP is approximately
(1-2) xtO- "C / cd. したがって。 It holds a charge of 5.03 XLO (C) per rt spacer. Assuming that
this is Ed, the total charge amount Q is given by Q = CV Q = Ed XN- +---(4) Formulas (2) and (3). If
the above-mentioned numerical value is applied to this equation (5), therefore, it is sufficient to
round up the fraction and provide 43 spacers, whereby the effective area of the diaphragm 4
becomes 5.6 XIO- '. As described above, according to the condenser microphone of the present
invention, a plurality of spacers having a surface area smaller than the surface area of the
diaphragm is provided on the diaphragm at appropriate intervals, and the diaphragm is
substantially small by these spacers. Since a plurality of diaphragms of the aperture are arranged
in parallel, the high frequency characteristics can be improved as compared with the
conventional condenser microphone of the same aperture, and the clearance between the
diaphragm and the back electrode plate Can be held. Furthermore, since the diaphragm is
supported on the back electrode plate at many points, the electrostatic adsorption force does not
act uniformly over the entire surface as in the conventional diaphragm, and the stability against
such adsorption is also improved. When the spacer is made of an electret material, a polarization
voltage can be obtained by this.
In this case, the electret is located between the diaphragm and the back plate. The upper and
lower ends of the electret will be short circuited. The lifetime can be extended.
Brief description of the drawings
FIG. 1 is a longitudinal sectional view schematically showing the internal structure of an
embodiment of a condenser microphone according to the present invention, FIG. 2 is a plan view
showing the diaphragm of FIG. 1, and FIG. 3 is a condenser microphone according to the present
invention And FIG. 16 is a characteristic diagram showing frequency characteristics of the
conventional and the condenser microphone.
In the figure, 4 is a diaphragm, 5 is a support ring, 6 is a back plate, and 13 is a spacer.
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