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DESCRIPTION JP2006238072

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DESCRIPTION JP2006238072
PROBLEM TO BE SOLVED: To provide an acoustic vibration generating piezoelectric bimorph
from which unnecessary high frequency components are removed without increasing the
number of parts and the volume in the configuration of an acoustic system to which a
piezoelectric bimorph element is applied. SOLUTION: By providing a blank portion where
electrodes are removed on the free end side of a piezoelectric bimorph 4, a resonance mode in a
high frequency region is suppressed, and an output of high frequency components is reduced,
thereby making it possible to obtain It is possible to provide an acoustic system in which the
howling phenomenon is less likely to occur without increasing the volume or the number of
parts. [Selected figure] Figure 5
Piezoelectric bimorph element for acoustic vibration generation
[0001]
The present invention relates to a speaker using a piezoelectric vibrator, and more particularly to
a piezoelectric bimorph element for generating acoustic vibration suitable for a small speaker or
bone conduction application.
[0002]
An electroacoustic apparatus generally consists of a mechanism that converts a change in
electrical quantity into mechanical and acoustic vibration, and conversely an acoustic and
mechanical vibration system and a mechanism that converts the vibration into a change in
electrical quantity.
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The action of the electromagnetism or the action of the piezoelectric effect has played an
important role in this energy conversion. When limited to the side that generates an acoustic
signal, the piezoelectric method generally has a configuration in which the stiffness of the
vibration system is high. In addition, it becomes an expensive apparatus if a mechanism that
generates a large amplitude is required in order to secure an output in the low range.
[0003]
Therefore, the mid-low range was not good in the piezoelectric system, but in recent years, a
system for efficiently exciting a large-area panel with the force of the piezoelectric was devised
by various devices. Sound output can also be obtained, and the application range is being
expanded. Although the piezoelectric vibrator can be designed relatively easily as a sound source
at a single frequency in the ultrasonic region, the means for obtaining acoustic vibration in the
audible sound region is generally limited, and a piezoelectric bimorph (also a piezoelectric
unimorph in the present invention) In most cases, it is necessary to use
[0004]
This is because the stiffness of the piezoelectric bimorph is structurally small and it is easy to
bring the resonance frequency into the audible range. Acoustic generation methods using
piezoelectric bimorphs can be roughly classified into two, and one has a structure with a single
piezoelectric bimorph element or a simple resonant vibrator, and a signal sound with only a
specific frequency or , Is an application as a high-frequency specialized speaker (tweeter).
Another one is a system which comprises a sound generation system with a vibration system
which combined a piezoelectric bimorph, a panel, and a case. The latter is capable of sound
output up to relatively low and medium frequencies. The shape of the piezoelectric bimorph
applied in the latter field is mainly a disk type (Patent Document 1), but a rectangular
piezoelectric bimorph is also used as needed (Patent Document 2). In an acoustic system using
this piezoelectric bimorph as a driving source, the acoustic characteristics and the frequency
characteristics of the generated vibrational force of the piezoelectric bimorph element are closely
related, and in order to obtain the targeted acoustic characteristics, the generated vibrational
force of the piezoelectric bimorph It is desirable that as many control means as possible be
required for the frequency characteristics of
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2
[0005]
Next, some explanation will be added regarding the vibration generating force of the cantilever
type piezoelectric bimorph. FIG. 1 shows basic frequency characteristics of the oscillating force
generated by the cantilever type piezoelectric bimorph. FIG. 2A shows an amplitude distribution
in the vicinity of the primary resonance frequency in the cantilever type piezoelectric bimorph.
FIG. 2 (b) shows the curvature distribution in the vicinity of the primary resonance frequency in
the cantilever type piezoelectric bimorph. FIG. 3 (a) shows the amplitude distribution in the
vicinity of the secondary resonance frequency in the cantilever type piezoelectric bimorph. FIG. 3
(b) shows the curvature distribution in the vicinity of the secondary resonance frequency in the
cantilever type piezoelectric bimorph. FIG. 4A shows an amplitude distribution near the third
resonance frequency in the cantilever type piezoelectric bimorph. FIG. 4B shows the curvature
distribution near the third resonance frequency in the cantilever type piezoelectric bimorph.
[0006]
When the piezoelectric bimorph is driven by an alternating current signal, a counteracting force
of vibration is generated at the fixed end. This acts in a direction orthogonal to the longitudinal
direction of the piezoelectric element, and has a basic frequency characteristic as shown in FIG.
Assuming that the longitudinal direction of the piezoelectric bimorph is an x-axis and the
direction orthogonal thereto is a y-axis, a value obtained by multiplying an individual mass point
SSdx in the longitudinal direction by the acceleration of that portion is an inertial force generated
in the y direction of that portion. The integral of ρSα dx from x = 0 to the length L is the
vibration generating force generated by the piezoelectric bimorph and acts in the y direction of
the fixed part. This is the vibration generating force of the piezoelectric bimorph shown in FIG.
[0007]
The three peaks shown in the figure are the primary, secondary and tertiary resonance
frequencies, respectively. Here, the frequency range to be driven is divided into the vicinity of the
primary resonance frequency and the vicinity of the secondary resonance frequency and the
third resonance frequency. In the region near the primary resonance frequency, 0 is shown at the
fixed end, and the maximum amplitude at the free end is shown in the distribution of FIG. 2A, and
the distribution of the curvature is at maximum near the fixed end and 0 at the free end. It shows
the distribution of FIG. 2 (b). The distribution of curvature is consistent with the distribution of
bending moments for the bimorph element. In the region near the second order resonance
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frequency, the distribution of the amplitude shows the distribution of FIG. 3 (a) whose direction
changes in the middle region between the center and the tip, and the curvature thereof becomes
the distribution of FIG. 3 (b). In the region near the third resonance frequency, the amplitude
distribution of FIG. 4A and the curvature distribution of FIG. 4B are obtained.
[0008]
In addition, there is Patent Document 3 using an electrode structure for the purpose of achieving
low drive voltage, large displacement change, and high stabilization of a piezoelectric actuator.
However, no method has hitherto been reported for adjusting the frequency dependence of the
vibration generating force for the above-mentioned piezoelectric bimorph from the electrode
structure.
[0009]
JP-A-11-215584 JP-A-2000-201398 JP-A-11-168246
[0010]
In a communication system (e.g., a hearing aid or a mobile phone) in which a speaker portion
generating acoustic vibration and a microphone coexisting with an audio signal, a basic problem
of howling phenomenon occurs.
This is a phenomenon of a communication system in which sound generated from a speaker
unnecessary to the microphone is propagated to the microphone through the air or a casing to
generate a kind of resonance phenomenon and a speaker emits a large sound. This howling tends
to occur easily at high frequencies above 3.5 kHz. Generally, low frequency transmission filters
are taken into the system to remove high frequency components unnecessary for
communication, but as a result there is a problem that the number of parts and the volume
increase. .
[0011]
The present invention has been made to solve the above-mentioned problems, and the technical
problem thereof is that an effective high frequency band can be obtained without increasing the
number of parts and the volume in the configuration of an acoustic system to which a
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piezoelectric bimorph element is applied. An object of the present invention is to provide a
piezoelectric bimorph element for acoustic vibration generation from which frequency
components are removed.
[0012]
A first invention for achieving the above object is a flat plate-like joined body in which a flat
plate-like piezoelectric member having polarizations and electrodes formed on both principal
surfaces is bonded to both sides of a flat plate-like elastic member. In the piezoelectric bimorph
element for generating acoustic vibration provided with a piezoelectric bimorph in which one
end of the joined body is fixed by a fixing member, an electrode portion formed on both main
faces of the piezoelectric member has an area of the main face of 100 It is a piezoelectric
bimorph element for acoustic vibration generation which reduced the electrode area equivalent
to 20 to 30% of area from the free end side of the above-mentioned piezoelectric body when it
is%.
[0013]
From the curvature distribution in FIG. 2 (b), it can be understood that the moment at the tip
portion is small in the frequency region near and below the primary resonance frequency, and it
is estimated that the influence on the vibration generating force is small in this frequency region.
When the frequency characteristics of the cut element were examined, even if the electrode was
removed to near 30% from the tip (free end), the third resonance frequency was not degraded to
the vicinity of the first and second resonance frequencies without degrading the performance of
the vibration generating force. It has been experimentally confirmed that the vibration
generating force in the vicinity can be greatly attenuated, and it is experimentally confirmed that
the output of the high region can be very effectively controlled when applied to a piezoelectric
bimorph for generating an acoustic vibration.
[0014]
As a result, the high frequency characteristics can be controlled without increasing the volume of
the piezoelectric bimorph and the performance of the vibration generating force, and it is
possible to provide extremely useful means for improving the performance of the acoustic
system as a vibration drive source of the piezoelectric bimorph. It became.
[0015]
The piezoelectric bimorph element for generating acoustic vibration according to the best mode
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for carrying out the present invention is manufactured by the following manufacturing method.
Hereinafter, a piezoelectric bimorph element for generating acoustic vibration according to the
best mode of the present invention will be described in detail with reference to the drawings.
[0016]
FIG. 5 is a cross-sectional view showing the configuration of a piezoelectric bimorph shim
material, a piezoelectric ceramic, and an electrode according to the best mode of the present
invention.
FIG. 6 is a view showing a fixing method of measurement relating to the vibration generating
force of the piezoelectric bimorph according to the present invention.
[0017]
Silver paste is printed on both sides of a thin plate (25 mm × 8 mm × 0.3 mm) of a piezoelectric
ceramic N10 material manufactured by NEC TOKIN.
Thereafter, the silver electrode is baked in the atmosphere at a temperature of 400 to 500 ° C.
for 20 to 40 minutes.
The silver paste of the electrode has a margin at the free end side in the length direction, and
when the area of the main surface of the piezoelectric ceramic is 100%, this margin portion
(portion where the electrode is reduced) is 0%, 10 Five types of samples (1), (2), (3), (4), and (5)
were prepared to be%, 20%, 25%, and 30%. Next, a polarization process of applying a DC voltage
of 600 V for 10 minutes at room temperature is performed on each sample. Align the two
piezoelectric ceramics in the same direction as the polarization direction. Then, using a pressing
jig, firmly bond the piezoelectric ceramic plate to the both sides of the shim plate (30 mm × 8
mm × 0.2 mm) made of 42 alloy material using a thermosetting epoxy adhesive, A piezoelectric
bimorph element is fabricated.
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[0018]
The piezoelectric bimorph 4 which is a unit of the cantilever beam structure which fixed one end
with the fixing jig 5 was produced. This unit is fixed to the vibration force sensor 6 as shown in
FIG. Thereafter, electrodes on both surfaces are connected to the same potential, an alternating
voltage of 30 V effective value is applied between the electrodes on the shim side, the frequency
is changed to 100 Hz to 10 kHz, and the frequency dependency of the vibration generating force
is measured.
[0019]
In the best mode for carrying out the present invention, the area of the margin portion [portion
where electrodes are reduced] on the free end side of the piezoelectric ceramic is preferably 25%,
and the area shape may be rectangular, It does not matter if it is circular.
[0020]
Embodiments of the invention will be described in detail in the drawings.
FIG. 7 shows the frequency dependency of the acoustic vibration generating force according to
the embodiment of the present invention. FIG. 8 shows the configuration of a prototype
piezoelectric bone conduction speaker. FIG. 9 shows the configuration of an experimental
apparatus for measuring howling phenomenon.
[0021]
From the results shown in FIG. 7, when the blanks with reduced electrodes are 10% (2) and 20%
(3), the overall output slightly decreases and the blanks without reduced electrodes are 0% (1)
The difference between the case of and the large resonance frequency dependency was not seen.
In the case (4) in which the margin with the electrodes was reduced is 25%, it was confirmed that
the output was significantly reduced in the vicinity of the third resonance frequency. In addition,
in the case of (5) in which the margin with reduced electrodes is 30% (5), although there was a
slight decrease in the overall output, it was also confirmed that the output was greatly attenuated
near the second resonance frequency.
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[0022]
Next, as shown in FIG. 8, a vibration system in which a piezoelectric bimorph was supported by a
cantilever beam structure inside the housing was configured to fabricate a speaker for bone
conduction. Using this speaker, an experimental apparatus as shown in FIG. 9 was combined with
an amplifier and a microphone to make a comparison of howling phenomena. When the distance
between the microphone and the prototype piezoelectric bone conduction speaker was changed
to face each other, it was investigated experimentally at what distance the howling phenomenon
occurs.
[0023]
As a result, the electrode reduced margins are 25% (4) and 30% compared to the cases where the
electrode reduced margins are 0% (1), 10% (2) and 20% (3). In the case of (5), it was found that
the distance at which the howling phenomenon occurs becomes significantly long. This result
suggests that the howling phenomenon is unlikely to occur. However, in the case of (5) where the
margin with reduced electrodes is 30% (5), the output of the speaker is attenuated more than in
the other cases, so it can not be said to be an effect due to only the high-order output reduction. I
understand. However, in the case of (4) where the blank space after electrode reduction is 25%, it
is presumed that the output near the third resonance frequency is suppressed.
[0024]
As described above, according to the present invention, the resonance mode in the high
frequency region is suppressed by providing the blank portion where the electrode is removed
on the free end side of the piezoelectric bimorph element, thereby reducing the output of the
high frequency component It has been confirmed that it is possible to provide an acoustic system
in which the howling phenomenon is less likely to occur without increasing the volume and the
number of parts of the piezoelectric bimorph element.
[0025]
The figure which shows the basic frequency characteristic of the oscillating force which a
cantilever type piezoelectric bimorph generate | occur | produces.
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The figure which shows the amplitude and curvature distribution of the 1st resonance frequency
vicinity in a cantilever type piezoelectric bimorph. Fig.2 (a) is a figure which shows amplitude
distribution of 1st-order resonant frequency vicinity in a cantilever type | mold piezoelectric
bimorph. FIG.2 (b) is a figure which shows the curvature distribution of primary resonance
frequency vicinity in a cantilever type | mold piezoelectric bimorph. The figure which shows the
amplitude and curvature distribution of secondary resonance frequency vicinity in a cantilever
type piezoelectric bimorph. Fig.3 (a) is a figure which shows the amplitude distribution of
secondary resonance frequency vicinity in a cantilever type | mold piezoelectric bimorph. FIG.3
(b) is a figure which shows the curvature distribution of the secondary resonance frequency
vicinity in a cantilever type | mold piezoelectric bimorph. The figure which shows the amplitude
and curvature distribution of the 3rd resonance frequency vicinity in a cantilever type
piezoelectric bimorph. Fig.4 (a) is a figure which shows the amplitude distribution of the 3rdorder resonance frequency vicinity in a cantilever type | mold piezoelectric bimorph. FIG.4 (b) is
a figure which shows the curvature distribution of the 3rd-order resonant frequency vicinity in a
cantilever type | mold piezoelectric bimorph. Sectional drawing which showed the shim material
of the piezoelectric bimorph which concerns on the best form of this invention, piezoelectric
ceramics, and the structure of an electrode. The figure which shows the fixing method of the
measurement regarding the vibration generation force of the piezoelectric bimorph concerning
this invention. The figure which shows the frequency dependency of the acoustic vibration
generation force which concerns on the Example of this invention. The figure which shows the
structure of the produced piezoelectric type bone conduction speaker. The figure which shows
the structure of the experimental apparatus for measuring the howling phenomenon.
Explanation of sign
[0026]
Reference Signs List 1 shim plate 2 piezoelectric ceramic plate 3 electrode 4 piezoelectric
bimorph 5 fixing jig 6 vibration force sensor 7 housing 8 microphone 9 bone conduction speaker
10 amplifier
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