JPH0591593

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DESCRIPTION JPH0591593
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the
propagation of air vibration having a frequency lower than the audible range which occurs when
a closed space defined in a house, a car or a safe room is suddenly opened. The present invention
relates to a low frequency sound detector suitable for detecting an elastic wave (hereinafter
referred to as a low frequency sound) which is a medium.
[0002]
2. Description of the Related Art A microphone is used to detect an audible sound that can be
heard by the human ear among elastic waves propagating through air as a medium, and an
ultrasonic wave is detected to detect an ultrasonic wave within about 20 kHz. The vessel is used.
FIG. 5 shows the structure of a ceramic microphone which is an example of the above-mentioned
conventional microphone, but this ceramic microphone is supported in a cantilever manner on a
main body 11 and a substrate 11 a provided inside the main body 11. A bimorph-type
piezoelectric element 12, a diaphragm 13 stretched over the main body 11 at an upper position
of the piezoelectric element 12, a pin connecting the central portion of the diaphragm 13 and the
free end of the piezoelectric element 12 And 14 are composed.
[0003]
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The audible sound wave vibrates the cone-like diaphragm 13 attached to the sealed main body
11, and this vibration is transmitted to the piezoelectric element 12 through the pin 14. Thereby,
the piezoelectric element 12 repeats bending deformation corresponding to the frequency of the
audible sound wave, and as a result, a piezoelectric output corresponding to the frequency of the
audible sound wave is generated in the piezoelectric element 12.
[0004]
However, such a conventional ceramic microphone, although having some sensitivity to ultra low
frequency sound, is originally a device for detecting audible sound, so it can be used in natural
environment. The various audible sounds that are present become noise on the contrary, and it is
a problem because it is not possible to selectively detect only very low frequency sounds.
Therefore, for example, it is virtually impossible to detect an infrasound (18 Hz or less) of lower
frequency than the audible range generated when the closed space in a house, a car or a safe
room is broken. there were. In addition, the destruction of the enclosed space in the house as
described above often occurs in a single shot, and does not occur continuously, which causes the
conventional microphone or ultrasonic detector to be It was difficult to use for detection of
frequency sound.
[0005]
In view of the above, it is an object of the present invention to provide an ultra low frequency
sound detector which maximizes sensitivity particularly to ultra low frequency sound and is not
influenced by an unnecessary signal caused by an audible sound or the like. To aim.
[0006]
SUMMARY OF THE INVENTION An ultra low frequency sound detector according to the present
invention comprises a container having an opening, a partition dividing the internal space of the
container into halves, and a piezoelectric element attached to the partition. A hole is formed in
the partition at a position corresponding to the opening, and the piezoelectric element is
supported in a cantilever manner on the periphery of the hole on the partition so as to be
disposed between the opening and the hole, The piezoelectric element is formed in the form of a
strip with a thickness of 300 μm or less provided with electrodes on both front and back sides
and has a planar area larger than the opening area of the opening and smaller than the area of
the hole, A vent having a predetermined opening area is bored along the inner wall of the
container in the connection portion between the container and the partition wall.
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[0007]
According to the present invention, the ultra-low frequency sound propagated inside the
container through the opening of the container causes the piezoelectric element to generate a
vibration corresponding to the frequency, and the vibration causes the piezoelectric element to
be vibrated by the vibration. The generated piezoelectric output is sent to an amplifier circuit
through an electrode and amplified.
In this case, in order to maximize the sensitivity of the piezoelectric element, it is necessary to
maximize the deflection of the piezoelectric element when the very low frequency sound reaches
the element surface of the piezoelectric element.
[0008]
In the present invention, since the strip-like piezoelectric element is cantilevered at one end
thereof at an appropriate position of the peripheral edge of the hole of the partition as described
above, the free end can be freely bent. Piezoelectric output can be obtained.
The sensitivity of the bending deformation of the piezoelectric element is improved as the
thickness of the piezoelectric element is thinner, and the sensitivity at that time is improved.
However, the mechanical strength is weakened and the forming process of the piezoelectric
element becomes difficult. The piezoelectric element requires at least a certain thickness or more.
This thickness can not be specified uniformly because the material and size of the piezoelectric
element are different, but the upper limit of the thickness for obtaining the sensitivity necessary
for detection of ultra low frequency sound is about 300 μm, and in particular About 100 μm is
preferable.
[0009]
The thickness of the piezoelectric element is set to be considerably thinner than that of the
piezoelectric element (12) used in the above-described microphone for detecting an audible
sound. Thus, making the piezoelectric element thin and strip-like improves the sensitivity by
directly receiving the ultra low frequency sound by the piezoelectric element itself, and at the
same time completely cuts off the audible sound that causes noise It is for. Although the
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thickness of the ceramic piezoelectric element itself is about 100 μm in the prior art, practically,
the ceramic piezoelectric element is always backed by a metal plate also serving as an electrode.
In the case of a strip-like configuration. And in a piezoelectric element backed by such a metal
plate, the thickness of the entire element usually reaches about several hundred μm.
[0010]
Furthermore, another way to maximize the deflection of the piezoelectric element is to allow the
pressure wave of the infrasound to be detected to reach only one side of the piezoelectric
element. That is, similar to the operation principle of the microphone described above, a so-called
pressure-type sound reception method is adopted, but this is done if a pressure wave of an ultralow frequency sound simultaneously reaches both sides of the piezoelectric element at the same
time. The reason is that the forces acting on the bending deformation of the piezoelectric element
due to these pressure waves cancel each other, and eventually the bending deformation of the
piezoelectric element does not occur. Therefore, according to the present invention, the
piezoelectric element is disposed at the corresponding position of the hole of the partition wall so
as to bisect the internal space of the container together with the partition wall, so only one side
of the piezoelectric element is the opening of the container They face each other, so that the
pressure wave of the infra-low frequency sound reaches only one side where it is applied.
[0011]
By the way, when the piezoelectric element is disposed at the position corresponding to the hole
of the partition wall as described above, such a structure makes it difficult to detect particularly
sensitive to ultra low frequency sound as it is. That is, in the arrangement structure of the
piezoelectric element, the surface opposite to the one surface of the piezoelectric element facing
the opening of the container, that is, one of the internal spaces of the container divided in half is
an enclosed space behind the piezoelectric element. In particular, in the case of an ultra low
frequency sound forming a so-called broad signal because of its configuration, a kind of damping
effect occurs to further lengthen the period of the response output, and as a result, the ultra low
to be detected There is a problem that it becomes impossible to distinguish frequency sound
from background noise.
[0012]
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According to the present invention, as described above, by forming a vent having a
predetermined opening area along the inner wall of the container and the partition wall, no
sealed space is created behind the piezoelectric element. An open space is formed behind the
piezoelectric element. As a result, the piezoelectric element is temporarily bent and displaced in
any direction by a direct impact of a single extremely low frequency sound, and then is displaced
in the opposite direction to restore from the bending deformation by the elasticity of the
piezoelectric element itself, and is at the equilibrium position. This is to enable stable damping
vibration. The effect of the vent can be most effectively enhanced by setting the gap between the
piezoelectric element and the hole formed in the partition wall to the minimum necessary size
that allows the bending deformation of the piezoelectric element. .
[0013]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the ultra low
frequency sound detector according to the present invention will be described below with
reference to FIGS. In the figure, 1 is a container having an opening 1a, 2 is a partition dividing
the internal space of the container 1 into an upper chamber 3 and a lower chamber 4, and 5 is a
hole opened in the partition 2 at the corresponding position of the opening 1a. Reference
numeral 6 denotes a unimorph-type piezoelectric element supported in a cantilever manner on
the partition wall 2 at the peripheral portion of the hole 5.
[0014]
The container 1 is, for example, a metal container having an inner diameter of 8 mm and a height
of 4.5 mm, and as shown in FIG. 2, the diameter of the opening 1a formed in the ceiling thereof is
set to 1.0 mm. . Further, the partition wall 2 is made of resin having a thickness of 1 mm, and is
installed and fixed at a height position of 2 mm from the bottom surface of the lower chamber 4.
[0015]
The hole 5 is provided at the central portion of the partition 2 and in this example is a square
hole of 2.2 × 2.2 mm. The piezoelectric element 6 has a PZT composition and is formed in a
strip shape having a thickness of 300 μm or less and has dimensions of 2.0 × 3.0 mm, and
silver having dimensions of 1.6 × 2.4 mm on both front and back sides The electrodes 6a and 6b
are attached and formed by a method such as vapor deposition. Then, the piezoelectric element 6
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supported in a cantilever manner as described above is made of epoxy resin or the like on the
partition wall 2 so that the length of the free end becomes about 2.1 mm and protrudes into the
area of the hole 5. Adhesively fixed to The dimensions of the hole 5 and the piezoelectric element
6 are set as described above, that is, the hole 5 is slightly larger than the shape of the
piezoelectric element 6, that is, the piezoelectric element 6 is disposed inside the hole 5
Therefore, when the piezoelectric element 6 is bent and deformed, it can be freely displaced. The
two conducting wires 7 connected to the piezoelectric element 6 are taken out of the container 1
in an airtight state and coupled to an amplifier with an amplification factor of 80 dB through a
shield wire.
[0016]
Further, a vent 8 having a predetermined opening area is bored along the inner wall of the
container 1 in the connection portion between the container 1 and the partition 2. (See FIG. 3 (a),
the shape of the vent 8 in this case is called i ), or the diameter of the partition 2 is set
slightly smaller than the inner diameter of the container 1 (Refer to FIG. 3 (b), and the shape of
the vent 8 in this case is referred to as "b"). In the present embodiment, the shape, size, and the
like of the vent 8 were variously set and tested.
[0017]
In Table 1 above, test NO. 4 and test NO. 5 shows a comparative example in which the vent 8 is
not provided. In the case of 4, the hole 5 is a square hole of 3.2 × 3.2 mm □. Moreover, test NO.
8 is a reference example in which the thickness of the piezoelectric element 6 is set to 400 μm
although the vent 8 is provided.
[0018]
Here, FIG. 4 shows the frequency characteristic of the amplifier coupled to the piezoelectric
element 6, but in the case of detecting the very low frequency sound generated with opening and
closing of this kind of closed space, the signal output value is noise In order to be clearly
distinguishable from the above, 1 Vp or more is required.
[0019]
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Next, a specific test performed using the ultra low frequency sound detector configured as
described above and the test results thereof will be described.
First, a warehouse of concrete structure 7 × 9 (floor) × 2.7 (height) m was selected as the
installation place of the detector, and 1.8 × 0.9 m provided in a suitable place such as a side wall
of the warehouse The door of the size of 45 mm was opened and closed at an opening / closing
speed of 0.2 m / s in the range of the opening / closing angle of 45 °. In this case, music of a
volume of 35 phones was simultaneously streamed as a noise source in the warehouse room, but
under such test conditions, an electret microphone (WM manufactured by Matsushita Electric
Industrial Co., Ltd.) which is an example of a microphone. The same test was conducted by
juxtaposing -063).
[0020]
The test results of this test are shown together as the signal output of the piezoelectric element 6
in Table 1, but it is apparent from Table 1 that the vent 8 is not provided (Test No. 4 and Test No.
5). The detector output of the sensor of the present invention is 1/2 or less as compared with the
ultra low frequency sound detector (Test No. 1 to 3 and Test No. 5, 6) according to the present
invention. Moreover, especially test NO. As apparent from the comparison with 8, it was
demonstrated that the thickness of the piezoelectric element 6 may be 300 μm or less. On the
other hand, in the test result in the case of the electret microphone, it was not possible to identify
the very low frequency sound only by detecting the signal for the audible sound. Also, when the
door opening and closing test was performed under quiet environmental conditions by stopping
the music as the noise source, the output of the electret microphone can not be distinguished
from the fluctuation of the bias level, Even in this case, it was not possible to identify the
infrasound.
[0021]
In order to determine the relationship between the thickness of the piezoelectric element 6 at
which 1 Vp or 1.3 Vp can be obtained as the piezoelectric output signal and the area of the vent
8 based on the above test results, a test similar to the above was performed. In this case, the
above test NO. The test was conducted under the same conditions as 1. The relationship between
the thickness of the piezoelectric element 6 and the area of the vent 8 obtained from the test
results is shown in Table 2 below. The numerical values described in the above Table 2 represent
the area (cm 2) of the vent 8.
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[0022]
In the above embodiment, particularly when the vent 8 (shape B) shown in FIG. 3 (b) is formed,
the partition wall 2 is formed by a column erected from the ceiling of the upper chamber 3 or the
bottom of the lower chamber 4. Can be supported at a predetermined position in the container 1.
However, the shape of the vent 8 is not limited to the above shape or shape, and various ones
having a predetermined vent area can be selected. . When the hole 5 is opened in the partition 2
at the corresponding position of the opening 1 a of the container 1, it is preferable to set the vent
8 around the partition 2 from the opening 4 a so as not to be seen linearly. This is advantageous
because it is possible to prevent the very low frequency sound from flowing from the upper
chamber 3 side to the lower chamber 4 side and offsetting the displacement of the piezoelectric
element 6. Even when the ventilation port 8 can be seen through to some extent, in this case,
there is a timing shift when the ultra low frequency sound reaches the front and back sides of the
piezoelectric element 6, and thus the piezoelectric element 6 as described above. The amount of
displacement of is substantially not a problem. Furthermore, although the above embodiment has
been described in the case of the unimorph form, it is of course possible to use a bimorph form
in which such unimorph forms are joined in a back-to-back manner.
[0023]
As described above, according to the very low frequency sound detector of the present invention,
the piezoelectric element can receive the impact directly from the very low frequency sound
flowing from the opening of the container and can make the maximum displacement. Thus, a
large piezoelectric output can be obtained from the piezoelectric element. On the other hand,
since there is no sensitivity to audible sound, there is an excellent advantage such as having a
sharp selectivity only to ultra low frequency sound. Moreover, the damping effect to the
piezoelectric element can be suppressed by the vent for air escape provided around the partition
in the container, and thereby the response characteristic of the piezoelectric element can be
improved and its sensitivity is improved. be able to.
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