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JP2010157886

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DESCRIPTION JP2010157886
To increase sound pressure in a low frequency region and a high frequency region in a
piezoelectric acoustic device. A piezoelectric acoustic device (1) includes a piezoelectric vibrator
(21), a plate (22) provided around the piezoelectric vibrator (21) and holding the piezoelectric
vibrator (21), a frame (23) supporting an outer peripheral portion of the plate (22) A resonator 3
is provided which resonates with the radiation sound emitted by the oscillator. The piezoelectric
vibrator 21 has a piezoelectric body 24 made of a piezoelectric element, and a metal plate 25
larger in diameter than the piezoelectric body 24 and concentrically attached to the surface of
the piezoelectric body 24. The plate 22 is made of a thin member which elastically holds the
piezoelectric vibrator 21 and has a bellows structure having a peak portion or a valley portion or
both in the circumferential direction. The amplitude of the piezoelectric vibrator 21 is increased
by the bellows structure of the plate 22, so that the sound pressure in the low and high regions is
increased. [Selected figure] Figure 1
Piezoelectric sound device
[0001]
The present invention relates to a piezoelectric acoustic device using a piezoelectric element.
[0002]
A piezoelectric acoustic device using a piezoelectric vibrator in which a piezoelectric element is
bonded to a metal plate has been known.
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1
Since this piezoelectric acoustic device has a thin and simple structure, it can be miniaturized and
is characterized by being inexpensive. However, such a piezoelectric acoustic device has a
problem that the sound pressure in the vicinity of the resonance frequency is high, but the sound
pressure in other frequencies, particularly in the low frequency region, is small. In the present
specification, the low frequency region (hereinafter referred to as the low band) refers to about
1000 Hz or less, and the high frequency region (hereinafter referred to as the high band) refers
to a region above about 1000 Hz. There is no strict boundary between
[0003]
There is also known a piezoelectric acoustic device in which the sound pressure in the low region
is increased by holding the piezoelectric vibrator by a plate made of resin (see, for example,
Patent Document 1). There is also known a piezoelectric acoustic device that increases the sound
pressure at an arbitrary frequency by attaching a metal for adjusting a resonance frequency to a
piezoelectric vibrator (see, for example, Patent Document 2). However, even in the piezoelectric
acoustic devices according to Patent Document 1 and Patent Document 2 as described above, the
sound pressure in the low band and the high band is still low. Patent Document 1: Japanese
Patent Application Laid-Open No. 9-271096 Japanese Patent Application Laid-Open No. 10126885
[0004]
The present invention has been made to solve the above-mentioned conventional problems, and
it is an object of the present invention to provide a piezoelectric acoustic device having a large
sound pressure in a low frequency region and a high frequency region.
[0005]
In order to achieve the above object, the invention of claim 1 is a piezoelectric vibrator
comprising a piezoelectric body comprising a piezoelectric element, and a metal plate having a
diameter larger than that of the piezoelectric body and concentrically attached to the surface of
the piezoelectric body. And a plate provided around the piezoelectric vibrator and holding the
piezoelectric vibrator, a frame supporting the outer peripheral portion of the plate, and a
resonator resonating with the radiation sound emitted by the piezoelectric vibrator. In the
piezoelectric acoustic device, the plate is made of a thin member which elastically holds the
piezoelectric vibrator, and has a bellows structure having a peak portion and / or a valley portion
in the outer peripheral direction, and the frame Is an open bottomed cylinder, the periphery of
the plate is supported by the inner wall of the cylinder to form a back air chamber between the
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plate and the bottom, and the resonator is configured to open the opening of the frame Provided
so as to cover the It is those that form the front air chamber between the door.
[0006]
The invention according to claim 2 is the piezoelectric acoustic device according to claim 1,
wherein the bellows structure is provided at a position near the frame of the plate.
[0007]
The invention according to claim 3 is the piezoelectric acoustic device according to claim 1 or 2,
further comprising a reflection plate provided around the opening of the frame and reflecting the
emitted sound forward, and the reflection plate is outside thereof. The peripheral portion has a
shape which is raised forward with a substantially exponential curve.
[0008]
According to a fourth aspect of the present invention, in the piezoelectric acoustic device
according to the third aspect, the resonator has a sound hole for passing the radiation sound, and
the sound hole has an opening position of the frame in the front-rear direction. It is provided
between the upper end position of the outer peripheral edge portion of the reflection plate.
[0009]
The invention according to claim 5 is the piezoelectric acoustic device according to any one of
claims 1 to 4, further comprising a plate-like horn cap for adjusting the directivity of radiated
sound in front of the resonator. It is.
[0010]
A sixth aspect of the present invention is the piezoelectric acoustic device according to any one
of the first to fifth aspects, further comprising: a duct connecting a front space of the reflection
plate and the rear air chamber; Is to be adjusted.
[0011]
According to the invention of claim 1, since the amplitude of the piezoelectric vibrator is
increased by the bellows structure of the plate, the sound pressure in the low range and the high
range is increased.
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[0012]
According to the invention of claim 2, the amplitude in the high region of the piezoelectric
vibrator is increased by the bellows structure of the plate, so that the sound pressure in the high
region is increased.
[0013]
According to the third aspect of the present invention, since the outer peripheral edge portion of
the reflecting plate is substantially an exponential curve, it is difficult for the radiated sound to
resonate at the outer peripheral edge portion, and the longitudinal direction and the lateral
direction of the reflecting plate The difference in the directivity of the emitted sound can be
reduced.
[0014]
According to the invention of claim 4, it is possible to further reduce the difference in the
directivity of the emitted sound in the longitudinal direction and the short direction of the
reflection plate.
[0015]
According to the invention of claim 5, since the transmission direction of the radiation sound is
widened by the horn cap, it is possible to make the directivity of the radiation sound dull.
[0016]
According to the invention of claim 6, since the resonance frequency can be provided in the low
band by the duct, the sound pressure in the low band can be increased.
[0017]
A piezoelectric acoustic device 1 according to an embodiment of the present invention will be
described with reference to FIGS. 1 to 6.
The piezoelectric acoustic device 1 according to the present embodiment includes a piezoelectric
speaker 2, a resonator 3 that resonates with the radiation sound emitted by the piezoelectric
speaker 2, a reflection plate 4 that reflects the radiation sound forward, and a housing that holds
them. It is equipped with five.
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The piezoelectric speaker 2 includes a piezoelectric vibrator 21, a plate 22 provided around the
piezoelectric vibrator 21 and holding the piezoelectric vibrator 21, and a frame 23 supporting an
outer peripheral portion of the plate 22.
The piezoelectric vibrator 21 has a piezoelectric body 24 made of a piezoelectric element, and a
metal plate 25 larger in diameter than the piezoelectric body 24 and concentrically attached to
the surface of the piezoelectric body 24.
The piezoelectric body 24 is, for example, lead zirconium titanate having a thickness of 0.05 to
0.1 mm.
The metal plate 25 is, for example, a 42 alloy (iron-nickel alloy containing 42% of nickel) having
a thickness of 0.05 to 0.1 mm, and it is desirable that the piezoelectric body 24 and the metal
plate 25 have the same thickness.
The piezoelectric body 24 and the metal plate 25 are attached, for example, by an epoxy
adhesive.
A silver electrode is provided on the surface of the piezoelectric body 24 and a lead wire (not
shown) is connected. By applying a signal voltage to the electrode, the piezoelectric body 24 is
distorted and its vibration is used as sound (vibration of air) Radiate.
[0018]
The plate 22 is a thin member that elastically holds the piezoelectric vibrator 21 and is, for
example, a resin film such as PEI (polyetherimide) or PEN (polyether naphthalate) having a
thickness of 75 to 188 μm.
The plate 22 has a donut shape, and the piezoelectric vibrator 21 is attached at the center by an
adhesive, and has a bellows structure in the outer peripheral direction.
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In this bellows structure, as shown in FIG. 6 (a), the peaks and valleys may be alternately formed,
or as shown in FIG. 6 (b), only the peaks may be formed. As shown in), the valley may be only.
[0019]
An example of a method of manufacturing the bellows structure of the plate 22 will be described
with reference to FIG.
The plate 22 in this example is a resin film and is molded by a heated mold.
First, as shown in FIG. 7A, the plate 22 is positioned between the mold A and the rubber material
B, and the mold A is heated to a predetermined temperature.
The mold A is processed into a bellows shape.
Next, as shown in FIG. 7 (b), the mold A is pressed against the rubber B with the plate 22
interposed therebetween. Next, as shown in FIG. 7 (c), the mold A is opened and the plate 22 is
removed. The plate 22 is formed into a bellows structure in accordance with the shape of the
mold.
[0020]
The frame 23 is, for example, a bottomed cylindrical body which is made of resin and one side is
open. The frame 23 adheres and supports the periphery of the plate 22 in the plane of the step
provided on the inner wall of the cylinder, and a rear air chamber 61 is formed between the plate
22 and the bottom surface. The resonator 3 is cap-shaped and has a sound hole 31 at the center,
is provided so as to cover the opening of the frame 23, and forms a front air chamber 62 with
the plate 22. The rear air chamber 61 and the front air chamber 62 reflect the radiation sound
emitted by the piezoelectric vibrator 21 to increase the sound pressure. The outer peripheral
edge 41 of the reflection plate 4 is raised in the forward direction.
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[0021]
The operation of the piezoelectric speaker 2 of the piezoelectric acoustic device 1 of the present
embodiment configured as described above emitting a radiation sound will be described with
reference to FIG. FIG. 8 shows the sound pressure of the piezoelectric speaker 2 with and without
the bellows structure of the plate 22. When a signal voltage of radiated sound is applied to the
piezoelectric body 24, the piezoelectric body 24 contracts and expands, but the metal plate 25 to
which the piezoelectric body 24 is attached does not contract and expand, so the piezoelectric
vibrator 21 bends. Do. The piezoelectric vibrator 21 repeatedly vibrates in this reverse bending
operation to generate a radiation sound. In the plate 22 having the bellows structure, the plate
22 is easily bent at the bellows structure, and the plate 22 is easily expanded and contracted in
the outer peripheral direction by the bending of the bellows structure. As a result, as shown in
FIG. 8, the amplitude of the piezoelectric vibrator 21 becomes large, and the piezoelectric
speaker is spread over the low frequency region (hereinafter referred to as low region) and the
high frequency region (hereinafter referred to as high region). The sound pressure of the
radiation sound emitted by 2 increases.
[0022]
The resonance frequency of the piezoelectric speaker 2 described above will be described with
reference to FIG. FIG. 9A shows a cross section of the piezoelectric speaker 2, and FIG. 9B shows
a model of the piezoelectric speaker 2. In FIG. 9 (a), the plate 22 is shown with the bellows
structure omitted. The piezoelectric speaker 2 can be regarded as a vibrating structure Q in
which a weight G is supported on a support P by a spring J, as shown in FIG. 9B. The resonance
frequency f of the vibrating structure Q is represented by f = 1 / (2π) · (k / m) <1/2>, where k is
the spring constant of the spring J and m is the mass of the weight G. Therefore, assuming that
the spring constant of the plate 22 is k0 and the mass of the piezoelectric vibrator 21 is m0, the
resonance frequency f0 of the piezoelectric speaker 2 is represented by the following formula: f0
= 1 / (2π) · (k0 / m0) <1/2> Ru. Then, assuming that the Young's modulus of the plate 22 is E,
the thickness of the plate 22 is h, and the diameter length of the plate 22 is L, the spring
constant k0 of the plate 22 is k0 = E · h <3> / L <2> Represented by / 4.
[0023]
In the piezoelectric speaker 2 without the bellows structure in FIG. 8 described above, the outer
diameter of the plate 22 was 53 mm, the diameter length L1 of the plate 22 was 7 mm, and the
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resonance frequency f1 was 180 Hz. On the other hand, in the piezoelectric speaker 2 having the
bellows structure, the outer diameter of the plate 22 was 50 mm, and the diameter length L2 of
the plate 22 was 6 mm. Since both of the piezoelectric speaker 2 without the bellows structure
and the bellows structure have the same Young's modulus E of the plate 22, the thickness h of
the plate 22 and the mass m0 of the piezoelectric vibrator 21, both of the piezoelectric speakers
2 with the bellows structure The ratio of the resonance frequency f2 of 2 to the resonance
frequency f1 of the piezoelectric speaker 2 without the bellows structure is f2 / f1 = L1 / L2 =
7/6. Therefore, the resonance frequency f2 is about 1.2 times the resonance frequency f1, and a
large sound pressure peak is formed around 210 Hz and 100 Hz. In such a piezoelectric speaker
2, the sound pressure can be increased by increasing the outer diameter of the plate 22, but
when the outer diameter of the plate 22 is restricted, as described above, the Young's modulus of
the plate 22 By changing the thickness and the diameter, the resonance frequency can be
changed to increase the sound pressure in an arbitrary frequency region.
[0024]
Next, the operation of the piezoelectric acoustic device 1 of the present embodiment configured
as described above will be described next. FIG. 10 shows the sound pressure at each frequency of
the piezoelectric acoustic device 1 with and without the resonator 3, and FIG. 11 shows the
structure of the resonator 3 and the formula for calculating its resonance frequency. The data
with the resonator 3 in FIG. 10 is data when the resonator 3 is configured such that the
resonance frequency fcav of the front air chamber 62 is 3000 Hz. The resonance frequency fcav
of the front air chamber 62 is the radius of the sound hole a, the length of the sound hole l, the
diameter of the front air chamber 62 d, the height of the front air chamber 62 h, and the sound
hole area S Assuming that the volume of the front air chamber 62 is V, the number of sound
holes is n, and the speed of sound is c, fcav = C / 2π × (S / V (l + 1.3a)) <1/2> = C / 2π × ( It
becomes 4na <2> / d <2> h (l + 1.3a)) <1/2>. The resonant frequency of the resonator 3 can be
adjusted by changing the configuration of the resonator 3. In the data of FIG. 10, the sound
pressure in the range of about 1000 to 4000 Hz is larger in the case of the presence of the
resonator 3 than in the case of the absence of the resonator 3. In the piezoelectric acoustic
device 1 of the present embodiment, since the above-described piezoelectric speaker 2 is
incorporated, the sound pressure in the low band and the high band is large, but by using such a
configuration, any resonator 3 can be used. The sound pressure of the frequency can be
increased.
[0025]
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First Modified Example Hereinafter, various modified examples of the present embodiment will
be described with reference to FIGS. 12 to 23. FIG. 12 shows a first modification. In this
modification, the bellows structure of the plate 22 is provided in the vicinity of the frame 23. FIG.
13 shows the sound pressure of the piezoelectric speaker 2 when the bellows structure is in all
the radial directions of the plate 22 and only in the vicinity of the frame 23. In this bellows
structure, as shown in FIG. 12 (a), the peaks and valleys may be alternately formed as in the
above-described embodiment, and as shown in FIG. 12 (b), the peaks are formed. However, as
shown in FIG. 12C, only the valleys may be used. When the bellows structure is only in the
vicinity of the frame 23, the sound pressure peaks in a high region (near 3000 Hz) as compared
with the case where the bellows structure is all in the radial direction of the plate 22. Thus, by
providing the bellows structure at a position near the frame 23 of the plate 22, the amplitude of
the piezoelectric vibrator 21 particularly in the high region becomes large, and the sound
pressure in the high region becomes large.
[0026]
Second Modified Example FIG. 14 shows a second modified example. In the present modification,
the plate 22 has a step-shaped portion 22 a in the portion holding the piezoelectric vibrator 21.
The inner diameter of the step-shaped portion 22 a is a size in which the piezoelectric vibrator
21 is fitted from the periphery, and the plate 22 is bonded to the piezoelectric vibrator 21 in a
state where the piezoelectric vibrator 21 is fitted. With such a configuration, the piezoelectric
vibrator 21 is securely attached to the plate 22, and the position at which the piezoelectric
vibrator 21 is attached becomes constant, so that the sound pressure and the resonance
frequency of the radiation sound emitted by the piezoelectric speaker 2 become stable.
[0027]
Third Modified Example FIG. 15 shows a third modified example. In the present modification, the
frame 23 has an L-shaped cross section 23 a at a portion supporting the plate 22. The L-shaped
cross section 23a has an L-shaped cross section in the vertical direction, and the plate 22 is
placed, fitted, and supported on the portion. The inner diameter of the L-shaped vertical portion
is sized to fit the plate 22 from the periphery, and the frame 23 is bonded to the plate 22 with
the plate 22 fitted. With such a configuration, the plate 22 is securely attached to the frame 23,
and the attachment position is fixed, so that the sound pressure and the resonance frequency of
the radiation sound emitted by the piezoelectric speaker 2 become stable.
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[0028]
Fourth Modified Example FIG. 16 shows a fourth modified example. In this modification, in
addition to the configuration of the third modification, the frame 23 further has a notch 23b on
one surface on which the L-shaped plate 22 is placed, and the adhesive C is a dispenser in this
42. It is filled from D. The applied adhesive C is deposited in the notches 23b and can be attached
without lifting the plate 22. Therefore, the plate 22 is securely attached to the frame 23, and the
sound pressure of the radiation sound emitted by the piezoelectric speaker 2 The resonance
frequency is stabilized.
[0029]
Fifth Modified Example FIG. 17A shows a fifth modified example. In this modification, the metal
plate 25 and the piezoelectric body 24 have a substantially disk shape, and the ratio of the radius
of the metal plate 25 to that of the piezoelectric body 24 is approximately 10: 7. FIG. 17B shows
a change in resonance frequency when the diameter of the metal plate 25 is fixed and the
diameter of the piezoelectric body 24 is changed. The piezoelectric body 24 and the metal plate
25 are circular, and the diameter of the metal plate 25 is 50 mm. The resonance frequency is
lowest when the diameter of the piezoelectric body 24 is around 35 mm, and the ratio of the
radius of the metal plate 25 to that of the piezoelectric body 24 at this time is 10: 7. The radius
ratio of the metal plate 25 to the piezoelectric body 24 is preferably between 10: 6 and 10: 8.
Therefore, by adopting the configuration as in this modification, the resonance frequency of the
piezoelectric speaker 2 is reduced, so that the sound pressure in the low range can be increased.
[0030]
Sixth Modified Example FIG. 18 shows a sixth modified example. In the present modification, the
plate 22 covers the piezoelectric vibrator 21, and an air layer E is provided between the plate 22
and the piezoelectric vibrator 21. The acoustic impedance of air is much smaller than the
acoustic impedance of the metal plate 25. Therefore, by providing the air layer E on the front
surface of the piezoelectric vibrator 21 by the plate 22, the acoustic impedance of the metal plate
25 can be relaxed. With such a configuration, the radiation sound emitted by the piezoelectric
vibrator 21 can be transmitted forward without being attenuated, so that the sound pressure can
be increased. In addition, it is possible to reduce the dip where the sound pressure sharply
decreases at a specific frequency.
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[0031]
Seventh Modified Example FIG. 19 shows a seventh modified example. In the present
modification, the outer peripheral edge portion 41 of the reflection plate 4 has a substantially
upright shape and has a shape that stands up forward. It is difficult for the radiated sound to
resonate in the portion of this exponential curve. In general, when the reflector 4 has a
substantially rectangular shape or a substantially elliptical shape, the directivity of the emitted
sound in the longitudinal direction and the latitudinal direction of the reflector 4 is different. By
making the outer peripheral edge portion 41 substantially an exponential curve, it becomes
difficult for the emitted sound to resonate at the outer peripheral edge portion, and the
difference in the directivity of the emitted sound between the longitudinal direction and the short
direction of the reflecting plate 4 is reduced. Can. At this time, the sound holes 31 of the
resonator 3 are further provided in the back and forth direction of the piezoelectric acoustic
device 1 between the opening position of the frame 23 and the upper end position of the outer
peripheral edge portion of the reflecting plate 4. The difference in directivity can be reduced.
[0032]
Eighth Modified Example FIG. 20 shows an eighth modified example. In the present modification,
the piezoelectric acoustic device 1 is provided in front of the resonator 3 with a plate-shaped
horn cap 7 for adjusting the directivity of the emitted sound. The horn cap 7 is curved in the
direction of the resonator 3 and is supported by a support 71 provided on the reflector 4. FIG. 21
shows the directivity of the emitted sound when the horn cap 7 is attached. The sound pressure
in the directions of 15 °, 45 ° and 90 ° is shown, where the forward direction of the
piezoelectric acoustic device 1 is 90 ° and the direction perpendicular to the forward direction
is 0 °. As described above, since the transmission direction of the radiation sound is broadened
by attaching the horn cap 7, the difference between the sound pressures in the directions of 15
° and 90 ° is reduced, and the directivity can be blunted. Furthermore, by changing the length
of the support 71, the directivity can be changed. When it is shortened, the directivity becomes
dull, and when it is lengthened, the directivity becomes sharp.
[0033]
Ninth Modified Example FIG. 22 shows a ninth modified example. In the present modification, the
piezoelectric acoustic device 1 includes a duct 8 connecting the front space of the reflecting plate
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4 and the rear air chamber 61, and the duct 8 adjusts the resonant frequency of the piezoelectric
acoustic device 1. The duct 8 is provided from the side surface of the cylinder of the frame 23 to
the bottom surface of the reflecting plate 4 and may be provided in plural. The duct 8 emits the
radiation sound that is reflected in the back air chamber 61 to the front of the reflecting plate 4.
The resonant frequency of the duct 8 can be changed by changing the cross-sectional area and
the length of the duct 8. Assuming that the resonant frequency fd of the duct 8 is D, the length of
the duct is L, the volume of the rear air chamber 61 is Vc, and r = (D / π) <1/2>, fd = It becomes
160 (D / Vc / (L + r)) <1/2>.
[0034]
FIG. 23 shows an example of the sound pressure of the piezoelectric acoustic device 1 with and
without the duct 8, and a part of the graph is shown enlarged. When the duct 8 is present, three
data in which the cross-sectional area of the duct 8 is different are shown. The shape of the duct
8 is limited by the shape of the piezoelectric acoustic device 1 and the approximate shape is
determined, so the resonant frequency of the duct 8 is mainly low. Also in the example in FIG. 23,
the sound pressure in the low range is large. Further, the peak frequency of the sound pressure
changes depending on the size of the cross-sectional area of the duct 8, and the peak frequency
moves to the higher frequency side as the cross-sectional area is larger. By making the
piezoelectric acoustic device 1 in such a configuration, the resonance frequency can be provided
in the low band, so that the peak frequency of the sound pressure in the low band can be
changed.
[0035]
The present invention is not limited to the configurations of the various embodiments described
above, and various modifications can be made without departing from the scope of the invention.
For example, in the above embodiment, the plate 22 is provided on the entire periphery of the
piezoelectric vibrator 21 to hold the piezoelectric vibrator 21. However, the plate 22 is provided
only on a part of the periphery of the piezoelectric vibrator 21. It is good also as composition.
[0036]
(A) is a block diagram of the piezoelectric acoustic device concerning an embodiment of the
present invention, (b) is a sectional view of the same piezoelectric acoustic device, (c) is an
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exploded perspective view of the same piezoelectric acoustic device. The block diagram of the
piezoelectric speaker in the same piezoelectric sound apparatus. Sectional drawing of the same
piezoelectric speaker. The block diagram of the piezoelectric vibrator of the same piezoelectric
speaker. (A) is a disassembled perspective view of the piezoelectric vibrator of the same
piezoelectric speaker, and a plate, (b) is a perspective view of a piezoelectric vibrator and a plate.
(A) thru | or (c) are sectional drawings of the plate of the same piezoelectric speaker. (A) thru | or
(c) are the figures which show the manufacturing method of the plate of the same piezoelectric
speaker in time series. The graph which shows the fluctuation | variation of the sound pressure
in the case with and without the bellows structure of a plate in the same piezoelectric speaker.
(A) is sectional drawing of the same piezoelectric speaker, (b) The model figure of the same
piezoelectric speaker. The graph which shows the fluctuation | variation of the sound pressure in
the case with and without a resonator in the same piezoelectric acoustic apparatus. The figure
which shows the calculation formula of the structure of the resonator in the same piezoelectric
acoustic apparatus, and its resonant frequency. (A) thru | or (c) are sectional drawings of the
plate in a 1st modification. The graph which shows the fluctuation of the sound pressure in the
plate. (A) And (b) is sectional drawing of the plate in a 2nd modification. Sectional drawing of the
plate and frame in a 3rd modification. (A) is a partial cross-sectional view of the frame in the
fourth modification, (b) is a cross-sectional view when the frame is filled with an adhesive, (c) is a
plane when the frame is filled with an adhesive Figure. (A) is a block diagram of the piezoelectric
vibrator in a 5th modification, (b) is a graph which shows change of resonant frequency when
changing the diameter of a piezoelectric material. Sectional drawing of the piezoelectric speaker
in a 6th modification. (A) is a block diagram of the piezoelectric acoustic device in a 7th
modification, (b) is a sectional view of the piezoelectric acoustic device. (A) is a block diagram of
the piezoelectric acoustic device in a 8th modification, (b) is a sectional view of the piezoelectric
acoustic device. The graph which shows the directivity of the radiation sound in the same
piezoelectric sound apparatus. Sectional drawing of the piezoelectric acoustic apparatus in a 9th
modification. The graph which shows the fluctuation | variation of the sound pressure in the case
with and without a duct in the same piezoelectric acoustic apparatus.
Explanation of sign
[0037]
Reference Signs List 1 piezoelectric acoustic device 21 piezoelectric vibrator 22 plate 23 frame
24 piezoelectric body 25 metal plate 3 resonator 31 sound hole 4 reflection plate 41 outer
peripheral portion 61 rear air chamber 62 front air chamber 7 horn cap 8 duct
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