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BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a partially cutaway perspective view of a
conventional microphone, FIG. 1 is a perspective view of the microphone unit of the same
microphone, FIG. 2 is a cross sectional view of another conventional example, FIG. 4 is a crosssectional view of a microphone according to an embodiment of the present invention, FIGS. 4 to
6 are cross-sectional views of another embodiment of the present invention, and FIG. In the
characteristic diagram, 11 ······· Case, 2 ···-frame, 3 ········· mW, 5.5 ′ ··································・ ・ ・ ・ ・
・ ・ ・ ・ ・ ・ ・ ・ Sound absorbing material.
DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a microphone
using a piezoelectric polymer film as a diaphragm, a microphone (which damps resonance Q of a
diaphragm of a rophone, flat sensitivity frequency characteristic It is an object of the present
invention to provide a microphone having the same. First, a conventional microphone of this type
will be described with reference to FIGS. 1a and 1b. In FIG. 1a, reference numeral 1 denotes a
case having an open upper surface, and a microphone unit i--non-λ rest f 2 ′ 12 shown in FIG.
1 is attached to the opening of the case 1-9. In this microphone unit, in a state where tension is
applied along the curvature of the curved frame 2, the piezoelectric polymer film 3 to be a
vibrating film is adhered. The microphone of this structure can design the reproduction
frequency limit and sensitivity of the high range by the curvature of the frame 2, that is, the
curvature of the diaphragm 30. That is, the resonance frequency fO and the sensitivity S, which
are suitable for the upper limit of the reproduction frequency, can be designed by the following
equation from the curvature radius γ of the diaphragm and the material constant of the
diaphragm. Where a and b are material constants of the piezoelectric polymer film which is a
vibrating film. However, in the conventional microphone structure shown in FIG. 1, although the
resonant frequency and sensitivity can be designed, the control of the Q factor of the resonance
is impossible. FIG. 2 shows another conventional example. This microphone is 5 in which the
sound absorbing material 4 is packed in the rear of the diaphragm 3 of the conventional
microphone shown in FIG. Although the control of the Q factor of this microphone can be
performed by 40 kinds of sound absorbing materials and the filling amount, it is very unstable
and difficult to design. The present invention is intended to eliminate the above-mentioned
conventional drawbacks, and a back plate having a hole is provided behind the vibrating
membrane, and the air flow generated by the vibration of the vibrating membrane is
concentrated in the hole opened in the back plate. A sound absorbing material is provided to
obtain the acoustic resistance, and the design of the Q value of the microphone can be easily
performed. One embodiment of the present invention will be described below with reference to
FIG. In FIG. 3, 1 is a case where the upper surface is open, 2 is a curved frame, 3 is a vibrating
membrane attached in a state where the frame 2 is tensioned, and this vibrating membrane 3 is
piezoelectric Electrodes are provided on both sides of the polymer film. Reference numeral 5
denotes a back plate having the same curvature as that of the frame 2, and the back plate 5 is
provided behind the vibrating membrane 3. 6 is a hole formed in the back plate 52 '). A sound
absorbing material 7 is filled in a space portion constituted by the case 1 and the back plate "2 5.
The air flow generated by the vibration of the vibrating membrane 3 by the sound pressure flows
through the holes 6 opened in the back plate 5 and flows into the cavitation filled with the sound
absorbing material 7. Since the velocity of the air flow through the holes 6 of the back plate 5 is
inversely proportional to the area of the holes 6, the flow velocity at the holes 60 is high, so that
the sound absorbing material 7 at the outlet of the holes 6 causes acoustic resistance. By
changing the aperture area of the back plate 5 in the microphone of this structure, the flow
velocity at the portion of the hole 6 can be varied to control the value of the acoustic resistance
generated by the sound absorbing material 7, thus controlling the Q factor of resonance It is
possible. FIG. 4 shows another embodiment, in which a sound absorbing material 7 is provided
on the rear surface of the back plate 6 so as to cover the hole e. FIG. 6 shows still another
embodiment, in which the sound absorbing material 7 is accommodated in the hole θ of the
back plate 5. Although the back plate having the same curvature as that of the frame 2 is used in
each of the above embodiments, a flat plate-like back plate 6 'having five holes 6 may be used as
shown in FIG. Further, the holes formed in the back plates 6 and 5 'are not limited to one, and a
plurality of holes may be formed. FIG. 7 shows the sensitivity frequency characteristics of the
microphone A of the present invention and the prior art B. According to the microphone Rohon A
of the present invention, since the Q of the resonance of the vibrating membrane can be damped,
it is flat compared to the conventional example. The required sensitivity frequency characteristic
can be obtained.