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BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an
acceleration sensor suitable for use in obtaining a motional feedback signal of, for example, a
motional feedback type speaker. SUMMARY OF THE INVENTION The present invention is an
acceleration sensor suitable for use, for example, to obtain a motional feedback signal of a
motional feedback type speaker, wherein an electret film is formed on either the diaphragm or
the back plate. In the acceleration sensor, which is arranged to detect acceleration, by providing a
through hole whose cutoff frequency of sound pressure sensitivity is 5 KHz or more in the
diaphragm, the sensitivity to sound pressure is lowered and the sensitivity to vibration is
relatively increased. The acceleration can be detected favorably. [Background Art]
Conventionally, a speaker of motional feedback (hereinafter referred to as "MFB") has been
developed as a speaker for improving transient characteristics and distortion. This MFB speaker
is configured, for example, as shown in FIG. The speaker shown in FIG. 5 is a so-called flat type
speaker using a flat diaphragm. In FIG. 5 (1), no yoke is provided, and a magnet (2) is disposed
around the yoke (1). A voice coil (3) is disposed at the center, and the upper portion of the voice
coil (3) is connected to a flat diaphragm (5) via a coupler (4). Further, the flat diaphragm (5) is
supported via the edge (8) at the tip of the frame (7) fixed to the plate (6) on the magnet (2).
Then, for example, an acceleration sensor (10), which is a component for MFB, is attached to the
central part of the flat diaphragm (5). With this configuration, the detection element on the
acceleration sensor (10) detects the acceleration of the diaphragm (5) and obtains the MFB signal
according to the detection. The MFB signal from the acceleration sensor (10) is fed back to the
front stage of the output amplifier (not shown) to control the vibration of the diaphragm (5) to
improve transient characteristics and distortion. By the way, as this acceleration sensor, for
example, using a piezoelectric sensor is proposed. In this piezoelectric sensor, for example, a socalled bimorph plate formed by sandwiching a thin brass plate with a ceramic plate and forming
a thin film layer of silver on the surface of the ceramic plate is pressed. The element is used as an
element, and an acceleration signal is obtained by a change in applied voltage due to a deflection
corresponding to the imaging movement of the piezoelectric element. [Problems to be Solved by
the Invention] However, such a piezoelectric sensor using a bimorph plate can not obtain
uniform detection characteristics unless the bimorph plate is accurately mounted on the
substrate with high accuracy, and it takes time for assembly. There was a problem.
There is also a sensor that obtains an MFB signal from a pick amplifier coil wound around the
same bobbin as a voice coil as an MFB signal detection element, but it is difficult to wind a pickup
coil close to a voice coil. As well as being susceptible to the influence of the temperature rise of
the speaker and the like. In order to solve this problem, for example, the electret MQ is disposed
close to the metal plate, and the electret film is vibrated by the vibration of the speaker so that
the charge can be extracted from the electret film, which is simple. It is also conceivable to detect
the acceleration signal with the following configuration, but the electret IN responds not only to
the vibration of the image pickup plate of the speaker but also to the sound pressure from this
speaker, so that the acceleration can not be detected favorably. was there. An object of the
present invention is to provide an acceleration sensor by which acceleration can be favorably
detected with a simple configuration. [Means for Solving the Problems] The acceleration sensor
according to the present invention, for example, as shown in FIG. In the acceleration sensor in
which the electret IQ is disposed on either one of (25) and back play 1- (26) to detect
acceleration, the cutoff frequency of the sound pressure sensitivity is 5 KHz or higher in the
diaphragm (25). A through hole (25a) is provided. [Operation] According to the acceleration
sensor of the present invention, the diaphragm (25) has a through hole (25a) having a cutoff
frequency of 5 Kllz or more for the sound pressure sensitivity, so the sound pressure escapes
from the through hole (25a). Thus, the vibration film (25) is lowered in sensitivity to sound
pressure, relatively increased in sensitivity to vibration, and acceleration can be detected
favorably. An embodiment of the acceleration sensor according to the present invention will now
be described with reference to FIGS. The acceleration sensor of this example is for obtaining an
MFB signal of an MFB speaker as in the prior art. That is, in FIG. 1, (23) shows an acceleration
sensor, and the acceleration sensor (23) is attached to one side of the diaphragm (5) of the
speaker as shown in FIG. To detect the acceleration according to And in this example, an
acceleration sensor (23) is comprised as shown in FIG. That is, the acceleration sensor (23) has a
small gap between the bottom surface (24a) of the case (24) and the diaphragm ring (27a) inside
the circular case (24) whose top surface is open, and the electret 19 (25) is placed, and a bank
plate (26) is placed on the top of the electret IIQ (25) with a slight gap through the diaphragm
ring (27b).
Then, one end of the lead wire (28) is soldered to the upper surface of the buff spray 1- (26).
Further, in the present embodiment, through holes (25a) having a diameter of about 100 μm are
provided at four places in the electret IQ (25) thus arranged, as shown in FIG. 2, for example. By
providing these four through holes (25a), the electret IPJ (25) does not respond to the sound
pressure of 5 K lz or less, and the vibration sensitivity A is smaller at <5 KHz or less as shown in
FIG. Higher than pressure sensitivity B In addition, the electret I! A through hole (26a) having a
larger diameter than the through hole (25a) of (25) is provided. Since the acceleration sensor of
this example is configured as described above, the gap g between the electret film (25) and the
back plate (26) is formed between the electret FJ (25) and the back plate (26). Of the capacitance
according to the size of Therefore, when the thin film electret film (25) vibrates due to the
vibration of the diaphragm (5) of the speaker to which the sensor (23) is attached, the gap g
changes, and the above-mentioned charge capacity changes. Then, the lead wire (28) connected
to the back plate (26) is connected to the gate of the FET (not shown), and the change of the
capacitance is taken out by the FET to change the voltage according to the vibration state. A
signal is obtained, and this voltage signal becomes an acceleration detection signal. By
constructing the sensor (23) capable of detecting the acceleration in this manner, the sensor (23)
is not influenced by the magnetic field from the speaker or the like, and is compact and
lightweight. Further, in the acceleration sensor (23) of the present example, by providing the
through hole (25a) in the electret / Ha (25) which is the vibrating film, it is possible to detect a
good acceleration. That is, like this, the electref l-I! When acceleration is detected by vibrating
(25), as shown by curve A in FIG. 3, the electret IN (25) can obtain an output characteristic with
substantially constant vibration sensitivity according to each frequency band. On the other hand,
electret M'A (25) responds not only to the vibration of the 41i moving plate (5) of the speaker
but also to the sound pressure emitted from the speaker etc., but the through hole (25a) The
sound pressure escapes from this through hole (25a) because it is in the electret film <25>. For
this reason, it does not react to the sound pressure below a certain frequency corresponding to
the size of the through hole (25a). For example, when the through hole (25a) is provided in the
state as described above, it does not respond to the sound pressure of <5 KHz or less as shown in
In the absence of the through hole (25a), the back pressure sensitivity becomes greater than the
vibration sensitivity in each frequency band as shown by the curve B 'in FIG. The vibration
sensitivity of 5 KHz or less is relatively increased because it does not respond to the sound
pressure of For this reason, since the MFB speaker improves the frequency characteristic of the
low frequency band of 5 KHz or less, acceleration of vibration of 5 K) lz or less necessary as the
MFB signal can be detected well. The size and number of the through holes (25a) of the electret
film (25) necessary for cutting the sound pressure of 5 KHz or less are the electret IQ! It differs
depending on the material and thickness of (25), and the above-mentioned example is just an
example. Further, the sound pressure may be cut at least 5 KHz or less, and the through hole (25
a) may be cut to a back pressure of 5 KHz or more as long as the vibration sensitivity is not
affected. In addition, it can also be set as the structure which provided the electref 1fJ in the back
plate (26) side. In short, the electret [130] is provided on the first and second song of hack-brae
(2ti) in Fig. 4 and between the electret film (30) and the bottom 1 'jj (24a) of the case (24), The
diaphragm (31) held by the diaphragm rings (27a) (27b) is disposed. This vibration IJ! As (31),
one obtained by coating a resin film with metal or the like is used, and a through hole (31a) is
provided at a predetermined place as in the case of the electret film (25) of FIG. ((T is configured
as in FIG. 1 example). With this configuration, since the vibration [2 (31) is not an electret film,
the freedom of material and thickness <N 曽, and the detection characteristic can be changed by
this vibration film (31). Furthermore, the present invention is not limited to the above-described
embodiment, and it goes without saying that various other configurations can be taken without
departing from the scope of the present invention. [Effect of the Invention] The acceleration
sensor according to the present invention has the vibration 19 (25) through which the cutoff
frequency of the sound pressure sensitivity is 5 Xh or more (',! Each time 5a) is provided, the
sound pressure escapes from the through hole (25a), the sensitivity to the sound pressure
decreases, the attitude to vibration relatively increases, and there is a benefit that good
acceleration can be detected.
Brief description of the drawings
FIG. 1 is a cross-sectional view of an embodiment of the acceleration sensor according to the
present invention, FIG. 2 is a cross-sectional view taken along the line II-II in FIG. 1, and FIG. FIG.
4 is a cross-sectional view showing a modification of the example shown in FIG. 1. FIG. 5 is a
cross-sectional view showing an example of the MFB speaker.
(23) is an acceleration sensor, (25) is an electret crotch, (25a) is a through hole, and (26) is a
back plate.
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