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JP2017112582

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DESCRIPTION JP2017112582
The present invention provides an ultrasonic microphone device which can easily measure and
display sound pressure in a high frequency band, and a method of converting an output signal of
an ultrasonic microphone from electrical unit of volt to pascal or decibel of acoustic unit. Do. An
ultrasonic microphone sensitivity measurement apparatus 500 used in an ultrasonic frequency
domain, which receives ultrasonic waves, has a unit of V / Pa or V / dB, or their inverse number.
It has ultrasonic wave reception sound pressure conversion means which has the reception
sensitivity for sound pressure conversion, and converts the output voltage from the ultrasonic
microphone 100 at the observation point into ultrasonic reception sound pressure based on the
reception sensitivity. [Selected figure] Figure 6
Ultrasonic microphone device and method for converting an output signal of an ultrasonic
microphone from electrical unit of volt to acoustic unit of Pascal or decibel
[0001]
The present invention relates to an ultrasonic microphone device and a method for converting an
output signal of an ultrasonic microphone from electrical unit of voltage to pascal or decibel of
acoustic unit.
[0002]
In recent years, techniques using ultrasonic signals are widely used in the fields of medical
diagnosis and nondestructive testing, and have become well-established as basic technologies
essential for the construction of medical fields or infrastructures for safe and secure society.
04-05-2019
1
Examples of application to the nondestructive inspection field of ultrasonic signals include
inspection of existence of cracks in infrastructure structures such as tunnels, expressways,
nuclear reactors, buildings, etc., and diagnosis of objects that can not be destroyed for inspection.
. This ultrasound technology has no concern about exposure to radiation like X-ray technology,
but the possibility of ultrasound exposure has been pointed out when using power ultrasound
applications that use nonlinear ultrasound and continuous waves. It is pointed out that if
ultrasound of a specific strength or higher is exposed to the human ear, it may affect the human
body in any way. With regard to sound pressure measurement, with regard to audible sound,
there is a sound level meter capable of displaying the magnitude of the sound at the
measurement point in dB, and it is possible to quantify the objective physical properties of the
sound. As for ultrasonic waves, hydrophones are commercially available as long as they can
measure the sound pressure at the observation point in water because the object is a living
organism and the acoustic characteristics are similar to water. On the other hand, what can be
expressed as sound pressure values for nonlinear acoustic waves called fractional harmonics
included in high-frequency aerial ultrasonic waves of, for example, several hundred kHz to
several MHz, is a dedicated measuring instrument using a method called balance method It is
known to use the method of mutual healing.
[0003]
Unexamined-Japanese-Patent No. 09-331599
[0004]
However, when measuring air ultrasonic waves using an apparatus using a balance method, there
is a restriction on the observation point position that the measurement point is a fixed point, that
is, the position of the balance plate with respect to the ultrasonic transmission source to be
measured.
In addition, although this device requires a windshield because the influence of the air flow
greatly affects the measurement accuracy to give a measurement error, such a limitation is not
suitable for the measurement for coping with the above-described ultrasonic exposure. In
addition, in the mutual rehabilitation method, three ultrasonic sensors are prepared and two of
them are selected, and each is used for transmission or reception, and a total of three sets of
transmission / reception measurement of the same method are performed to drive at the time of
transmission This is a method of estimating and calibrating the reception sensitivity from the
04-05-2019
2
relationship between the voltage and the reception voltage at the time of reception. However, in
the case of using the mutual healing method, it is necessary to make the aperture size equal to or
less than the wavelength of the ultrasonic wave, for example, 0.85 mm or less in the case of 400
kHz. In addition, the mutual rehabilitation method needs to combine transmission sensitivity
sufficient for transmission and reception sensitivity sufficient for reception, but the aperture size
is so small that there is no piezoelectric material that can satisfy the condition of sensitivity. .
[0005]
Therefore, the present invention provides an ultrasonic microphone device that is easy to
measure and enables display of sound pressure in a high frequency band.
[0006]
The ultrasonic microphone device of the present invention is an ultrasonic microphone device
used in the ultrasonic frequency domain, and the ultrasonic microphone receiving ultrasonic
waves has a unit of V / Pa or V / dB, or their inverse number. It has a receiving sensitivity for
sound pressure conversion as a unit, and has ultrasonic receiving sound pressure converting
means for converting the output voltage from the ultrasonic microphone at the observation point
into ultrasonic receiving sound pressure based on the receiving sensitivity. It is characterized by
The ultrasonic microphone device can enhance the convenience of the user by being able to
display the ultrasonic wave with a sound pressure of Pa or dB so that the user can visually
recognize.
[0007]
(2) The ultrasonic reception sound pressure conversion means of the present invention collates
the output voltage V meas (V) of the ultrasonic microphone with the reception sensitivity S rec
stored in the memory of the ultrasonic reception sound pressure conversion means The received
sound pressure P cal (Pa) may be calculated by dividing the output voltage V meas (V) by the
reception sensitivity S rec.
[0008]
(3) The reception sensitivity of the present invention may be created through the following steps:
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First step: With an opening radius a, frequency nf 0 (n is an integer) when propagating to the air
An ultrasonic standard sound source serving as a broad band ultrasonic piston sound source for
generating and propagating a fundamental ultrasonic wave of frequency f 0 which produces high
order harmonics of
Second step: The vibration velocity u 0 of the surface of the ultrasonic standard sound source or
the sound source sound pressure P 0 obtained by integrating the acoustic impedance to the
vibration velocity u 0 is measured. Third step: using the vibrational velocity u 0 or the source
sound pressure P 0 measured in the second step, the aperture radius a, and the frequency f 0, the
ultrasonic standard using the Rayleigh integral equation The sound pressure P cal fund of the
fundamental wave formed by the sound source is calculated in units of pascal (Pa) or decibel
(dB), or using the nonlinear differential equation of KZK, the ultrasonic standard sound source
forms the fundamental wave The sound pressure P cal fund, the sound pressure P cal 2nd of the
second harmonic component, and the sound pressure P cal # 3rd of the third harmonic
component are calculated in units of Pascal (Pa) or decibel (dB). Fourth step: With respect to the
sound field calculated in the third step, create a table showing the relationship between sound
pressure and position. Fifth step: The ultrasonic microphones are sequentially scanned at each
position set in the table created in the step 4 to measure the output voltage V meas of the
ultrasonic microphone for each position. Sixth step: The output voltage V meas of the ultrasonic
microphone measured in the fifth step is a fundamental wave component V meas fund, a second
harmonic component V meas 2nd, and a third harmonic component V meas 3rd , And extract the
voltage level value of each frequency component. Seventh step: sound pressure of the
fundamental wave component, sound pressure of the second harmonic component, and sound
pressure P cal 2nd of the third harmonic component at each position calculated in the fourth
step P cal 3rd and the fundamental wave component of the output voltage at the measured
position of the fifth step corresponding to each position, the second harmonic component and
the third harmonic component V meas fund, V meas 2nd, P meas Based on 3rd, each reception
sensitivity Vmeasfund / Pcalfund, Vmeas2nd / Pcal2nd and Vmeas3rd / Pcal3rd are calculated.
Eighth step: of the reception sensitivity Vmeasfund / Pcalfund, Vmeas2nd / Pcal2nd, and
Vmeas3rd / Pcal3rd of the fundamental wave component, the second harmonic component and
the third harmonic component respectively The coincidence is evaluated, and the average value
of the measured value and the calculated value of the reception sensitivity which has a difference
of 5% or less from the coincident case is calculated, and this average value is taken as the
reception sensitivity of the ultrasonic microphone.
The theoretical sound field pattern of the relationship between the reference sound source
specification (aperture radius, frequency, vibration velocity on the ultrasonic wave transmission
surface) and the relative coordinates of the observation point is obtained by the operation of the
procedure of the first step to the eighth step. Measured sound in units of volts (V), calculated
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using unit Pascal (Pa) or decibel (dB), and setting the relationship between the reference sound
source and the observation position to be the same condition as in theoretical calculation. By
measuring the field and evaluating the coincidence (correlation coefficient) in the acoustic focus
area of both patterns, it is judged that the same physical quantity of both is observed, and the
unit of the ultrasonic microphone is (V / Pa Or (V / dB) is obtained. Then, the theoretical sound
field and the measured sound field at a plurality of points in the acoustic focus area having the
least error factor (i.e., a plurality of points sandwiched by positions (two points) which become
1/2 the peak value) The coincidence is evaluated by the correlation coefficient between the
calculated value and the measured value at a plurality of positions sandwiched by the positions
(two points) which are 1/2 value of the peak value. Thus, it is possible to provide an ultrasonic
microphone with a reliable sensitivity indication. The KZK non-linear differential equation means
(Khokhlov-Zabolotskaya-Kuznetsov) non-linear differential equation. Hereinafter, the same
applies in the present application.
[0009]
(4) The ultrasonic microphone of the present invention has a porous polypropylene film on
which gold is formed on both sides as an acoustoelectric conversion element for converting
received ultrasonic waves into an electric signal, and the porous polypropylene film is charged
The piezoelectric electret may be used. As an acoustoelectric conversion element which is the
heart of an ultrasonic microphone, a piezoelectric electret vibrator charged with a porous
polypropylene film exhibiting wide band, high sensitivity and high SN (signal / noise ratio)
characteristics is used. It is possible to measure with high reliability the ultrasonic sound field of
[0010]
(5) The ultrasonic microphone of the present invention may have a protective film layer. The
present invention forms a weather-resistant protective film on the ultrasonic wave receiving
surface, so it becomes an ultrasonic microphone that is particularly resistant to the environment
against moisture and dust.
[0011]
(6) The ultrasonic microphone of the present invention and a source follower type impedance
converter connected to the ultrasonic microphone may be accommodated in a housing. The built-
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in impedance conversion circuit of the source-follower type FET input is placed close to the
piezoelectric electret vibrator, so it is not affected by the length of the cable to be connected, the
intrinsic impedance or the capacitance, and the influence of the incident noise. Also, it becomes
possible to provide an ultrasonic microphone with a good SNR.
[0012]
(7) The ultrasonic wave received sound pressure conversion means connected to the impedance
conversion unit of the present invention may be accommodated in the housing. Since the
functions for all the steps described in claim 3 are integrated in the ultrasonic microphone
housing, it is possible to provide the sound pressure of the direct acoustic unit to the user.
[0013]
(8) The method of converting the output signal of the ultrasonic microphone of the present
invention from electrical unit of volt to acoustic unit of Pascal or decibel has an aperture radius a
and frequency nf 0 (n is an integer) when propagating to the air A first step of providing an
ultrasonic standard sound source serving as a broadband ultrasonic piston sound source for
generating and propagating a fundamental ultrasonic wave of frequency f 0 which produces
high-order harmonics, and vibration velocity u of the surface of the ultrasonic standard sound
source 0 or 2nd step which measures sound source sound pressure P0 which accumulated
acoustic impedance to this vibration speed u 0, said vibration speed u 0 or said source sound
pressure P 0 measured at said 2nd step, The opening radius The sound pressure P cal fund of the
fundamental wave formed by the ultrasonic standard sound source is calculated in units of Pascal
(Pa) or decibel (dB) using a and the frequency f 0 using Rayleigh's integral equation Sound
pressure P cal fund of the fundamental wave formed by the ultrasonic standard sound source,
sound pressure P cal 2nd of the second harmonic component, and sound pressure of the third
harmonic component using the nonlinear differential equation of KZ or KZK A third step of
calculating P cal 3rd in units of Pascal (Pa) or decibel (dB), and a table showing the relationship
between sound pressure and position for the sound field calculated in the third step A fifth step
of measuring the output voltage V meas of the ultrasonic microphone for each of the positions by
sequentially scanning the ultrasonic microphones to each position set in the table created in the
step; The output voltage V meas of the ultrasonic microphone measured in five steps is separated
into a fundamental wave component V meas fund, a second harmonic component V meas 2nd,
and a third harmonic component V meas 3rd Power of frequency component Sound pressure of
the fundamental wave component at each position calculated in the fourth step, and sound
pressure of the second harmonic component and sound pressure of the third harmonic
component at each position calculated in the fourth step The fundamental wave component, the
second harmonic component, and the third harmonic component V meas of P cal fund, P cal 2nd,
04-05-2019
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P cal 3rd, and the output voltage at the measurement position of the fifth step corresponding to
each position The seventh step of calculating the respective reception sensitivities Vmeasfund /
Pcalfund, Vmeas2nd / Pcal2nd and Vmeas3rd / Pcal3rd based on the fund, Vmeas2nd,
Vmeas3rd and the fundamental wave The consistency of the reception sensitivity V meas fund /
P cal fund, V meas 2nd / P cal 2nd, and V meas 3rd / P cal 3rd of each of the component and the
second harmonic component and the third harmonic component is evaluated, Calculate the
average value of the measured value and the measured value of the reception sensitivity that has
a difference of 5% or less from the matched case, and this average And characterized by having a,
a eighth step of the reception sensitivity of the ultrasonic microphone.
[0014]
The ultrasonic microphone device of the present invention is effective in that measurement is
easy and sound pressure can be displayed in a high frequency band.
[0015]
It is a sectional view showing one embodiment of an ultrasonic microphone device concerning
the present invention.
It is a schematic diagram of the piezoelectric electret of the ultrasonic microphone apparatus
which concerns on this invention.
It is a schematic diagram showing the ultrasonic wave reception principle of the piezoelectric
electret of the ultrasonic microphone device concerning the present invention. It is a schematic
diagram showing the ultrasonic wave reception principle of the piezoelectric electret of the
ultrasonic microphone device concerning the present invention. It is a circuit diagram showing
an impedance conversion circuit of an ultrasonic microphone device concerning the present
invention. It is a figure showing the ultrasonic microphone sensitivity measuring device of the
ultrasonic microphone device concerning the present invention. It is the flowchart which showed
the procedure for determining the receiving sensitivity of the ultrasonic microphone of the
ultrasonic microphone apparatus concerning the present invention. It is a graph which shows the
agreement with the underwater actual sound field and the sound field on calculation. It is a graph
which shows the agreement with the air measurement sound field and the sound field on
calculation. (A) is sectional drawing which shows the other modification of the ultrasonic
microphone apparatus shown in FIG. 1, (b) is sectional drawing which shows the other
modification of the ultrasonic microphone apparatus shown in FIG.
04-05-2019
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[0016]
Hereinafter, preferred embodiments of an ultrasonic microphone device used in an ultrasonic
frequency region (for example, several hundred kHz or more and several MHz or less) according
to an embodiment of the present invention will be described in detail with reference to the
drawings.
[0017]
As shown in FIG. 1, the ultrasonic microphone device 1 includes an ultrasonic microphone 100
capable of receiving ultrasonic waves, a source follower type impedance conversion unit 126
connected to the ultrasonic microphone 100, and a signal processing circuit. And an ultrasonic
reception sound pressure conversion unit (ultrasound reception sound pressure conversion
means) 128.
The ultrasonic microphone 100, the impedance conversion unit 126, and the ultrasonic reception
sound pressure conversion unit 128 are accommodated in a conductive housing 107.
[0018]
In the ultrasonic microphone 100, the piezoelectric electret 101 disposed in the opening 107 a
on the front side of the housing 107, the electrodes 102 and 103 provided in close contact with
both surfaces of the piezoelectric electret 101, and the opening on the back side of the
piezoelectric electret 101 A metal backing member 104 joined to the electrode 103 via the
adhesive layer 106 at the portion 107 b. In the opening 107 a of the housing 107, the resin
protective film layer 105 in close contact with the electrode 102 is disposed, and the opening
107 b of the housing 107 is closed by the cap 108 having conductivity.
[0019]
An insulating layer 112 is formed on the inner wall surface of the housing 107 so that the
electrode 102 and the electrode 103 and the metal backing member 104 do not short. The
electrode 102 can be electrically connected to the housing 107 and the cap 108 via a conductive
04-05-2019
8
member 110 such as a lead wire, and is connected to the impedance conversion unit 126
through a wire 119 b.
[0020]
The electrode 103 can be electrically conducted to the backing member 104 through the
insulating thin film adhesive layer 106, and is connected to the impedance converter 126
through the wire 119a.
[0021]
That is, in the present embodiment, the piezoelectric electret 101 uses the λ / 4 resonance mode
so that the response frequency band is 1⁄2 of that in the λ / 2 resonance mode.
For this purpose, the back side of the piezoelectric electret 101 fixed rigidly by the metal backing
member 104 is not displaced, and the interface between the surface of the backing member 104
and the piezoelectric electret 101 is a nodal point of vibration. The surface side of the electret
101 is made to exhibit the maximum vibration amplitude Amax as displacement free. For this
purpose, for the adhesive layer 106, an epoxy adhesive which has a small viscosity before curing
and which is thin and hard after bonding is selected. When an epoxy adhesive is employed, the
adhesive layer 106 is electrically conductive in a direct current manner by pressure curing, so
the signal cable 119 may be soldered to the metal backing member 104.
[0022]
FIG. 2 is a view schematically showing an internal sectional structure of the piezoelectric electret
101. As shown in FIG. As shown in FIG. 2, the piezoelectric electret 101 is a member provided
with a porous polypropylene film. As shown in FIG. 2, in the piezoelectric electret 101
sandwiched by the electrodes 102 and 103 (see FIG. 1), fine flat voids 302 are uniformly
distributed at a volume ratio of about 50%. Also, the piezoelectric electret 101 is positively and
negatively charged so as to surround the flat void 302 by charging means such as corona
discharge.
[0023]
Hereinafter, the one charging unit 320 is conveniently referred to as a charging dipole, and the
04-05-2019
9
action force on the electric field is hereinafter referred to as a charging dipole moment for
convenience, and the amount thereof is represented by μ, μ = q · z (q : Charge amount, z: short
axis distance of flat void.
[0024]
The charging dipole moment μ is integrated by the number N of flat voids 302 in the entire
piezoelectric electret 101, and the entire piezoelectric electret 101 becomes N μ.
This is conveniently referred to as charge polarization P. This charged polarization P is P = Nμ =
N · q · z (where, z is the minor axis distance of the flat void). It is represented by). The charge of
the charge polarization P is not observed from the outside because it is bound and neutralized by
the bound charge localized near the surface of the piezoelectric electret 101 as in the case of a
normal piezoelectric material. The situation is shown in FIG.
[0025]
In order to make the following description easy to understand, it is assumed that the charge
amount of the total charge polarization P is +10 on the upper side 309 and −10 on the lower
side 310. As long as there is no change in the external environment such as pressure,
temperature, atmospheric pressure, etc., there is a balance between the negative binding charge 10 and the positive binding charge +10 at the electrodes 102 and 103, respectively. No charge is
observed on 103.
[0026]
FIG. 4 shows this state together with the flat void 302. In such a neutralized state, ultrasonic
waves are incident, and as shown in FIG. 4, when the electrode 102 on the upper surface side is
compressed and deformed by δz (see reference numeral 315), the charge polarization P is
expressed by the equation P = N · q · z According to, ΔP = N · q · δz decreases.
[0027]
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10
For example, when the charge amount of the charge polarization P is changed to +8 on the upper
surface side 309 and to -8 on the lower surface side 310 by compressive deformation, the bound
charge can not immediately change in state. Therefore, on the upper surface side 309, the charge
amount of the difference +2 on the lower surface side 310 with the difference of -2 will be
focused on each of the electrodes 102 and 103. The focusing charge Q is detected as a voltage
value at V = Q / C, where C is the capacitance of the piezoelectric electret 101. Alternatively, the
charge state is detected by the charge amplifier and converted into a voltage signal. This is the
output voltage of the ultrasonic microphone 100.
[0028]
Since the aforementioned bound charge is localized at the interface between the electrodes 102
and 103 and the piezoelectric electret 101, the adhesion between the electrodes 102 and 103
and the piezoelectric electret 101 is important. When the piezoelectric electret 101 is wrinkled
or warped due to the electrode formation, stable acoustoelectric conversion can not be
performed, so that deformation such as warpage that affects the adhesion of the electrodes 102
and 103 and the deformation of the flat void 302 is not generated. It is preferable to devise. In
this embodiment, Au electrodes are formed on both surfaces of the piezoelectric electret 101 by
optimizing the pretreatment of the surface of the piezoelectric electret 101 and the sputtering
conditions.
[0029]
As shown in FIG. 1, the ultrasonic wave reception sound pressure conversion unit 128 is
connected to the impedance conversion unit 126 configured by a circuit by the wiring 127, and
converts the output voltage from the impedance conversion unit 126 into ultrasonic wave
reception sound pressure. Signal processing circuit. The internal circuit of the ultrasonic
reception sound pressure conversion unit 128 can obtain an analog / digital conversion circuit
ADC, a fast Fourier transform circuit FFT, and three-peak frequency characteristics, so the
maximum value at each peak can be A1, Assuming A2 and A3, A1 is the fundamental wave
component of the receiving ultrasonic wave and its peak frequency is f 0, A2 is the second
harmonic component of the receiving ultrasonic wave and its peak frequency is 2f 0 and A3 is
the third of the receiving ultrasonic wave The peak frequency of the harmonic component is 3f 0.
04-05-2019
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[0030]
The parameters of these A1 @ f0, A2 @ 2f0, A3 @ 3f0 can be easily extracted by digital operation.
These values are compared with the position information from the position sensor provided in
the vicinity of the ultrasonic microphone, that is, the coordinate information (xi, yj, zk), and the
position function A1 (xi, yj, zk) with the maximum amplitude, A2 ( xi, yj, zk), A3 (xi, yj, zk) are
determined by the sound field forming device. The units of A1, A2 and A3 which are actual
measurement values are all (V).
[0031]
The ultrasonic reception sound pressure conversion unit 128 incorporates a memory.
Specifically, in the ultrasonic reception sound pressure conversion unit 128, the Rayleigh integral
expression and / or the KZK non-linear differential equation expressed in sound pressure units
[Pa] or sound pressure level units [dB] in advance are used. For sound pressure conversion
obtained from the relationship between the calculated sound field data calculated using, the
measured sound field data obtained from the ultrasonic microphone 100, the calculated sound
field data, and the measured sound field data Reception sensitivity data S1 (xi, yj, zk), S2 (xi, yj,
zk) and S3 (xi, yj, zk) of the reception sensitivity (unit: V / Pa) are stored. Note that Rayleigh's
integral equation can calculate the calculated sound field data of the fundamental wave, and the
non-linear differential equation of KZK can calculate the calculated sound field data of the
fundamental wave, the second harmonic and the third harmonic. Can.
[0032]
As shown in FIG. 5, the impedance conversion unit 126 uses an FET (field effect transistor) 215
having a high input impedance at the first stage of the circuit. To the gate terminal 212 of the
FET 215, a signal cable 119 for signal reception of the piezoelectric electret 101 and a gate
resistor 213 with an extremely high resistance of GΩ are connected. The drain 118 of the FET
215 is provided with a drain terminal 216. The source resistance 214 is connected between the
source 210 of the FET 215 and the ground, and an output signal is obtained from the source
terminal 217 of the FET 215.
[0033]
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12
The piezoelectric electret 101 has a feature that the dielectric constant is extremely small.
Therefore, in the piezoelectric electret 101, the impedance represented by 1 / 2πfC (f: operating
frequency, C: electrostatic capacitance of the piezoelectric electret 101) becomes extremely large,
and electrical matching with the input impedance of the subsequent circuit such as an amplifier
can not be achieved. Therefore, multiple reflection noise will be placed on the input part of FET
215, and if the input impedance is low and amplification function is given as the reception first
stage circuit, the noise is amplified and the output signal to the subsequent circuit is extremely
SN It becomes a bad signal and becomes impractical. By using the source-follower type
impedance converter 126 shown in FIG. 5, the impedance matching and the output impedance
can be converted to a low level, so that electrical matching with the subsequent circuit can be
achieved, and no amplification is performed. There is a merit that it can be made wide.
[0034]
Next, a measurement procedure and a measurement device for the sound pressure conversion
reception sensitivity (unit: V / Pa) of the ultrasonic microphone 100 will be described below.
[0035]
As shown in FIG. 6, the ultrasonic microphone sensitivity measurement device 500 has a
manipulator 507 capable of moving the ultrasonic microphone 100 in the x direction 509, the y
direction 510, and the z direction 508, and a desired frequency toward the ultrasonic
microphone 100. And a position sensor 511 for detecting the position of the ultrasonic
microphone 100 moved by the manipulator 507.
[0036]
The ultrasonic microphone 100 is suspended from the manipulator 507 in a state of being held
by the holder 505 (first step).
The output of the signal generator 512 is amplified by the power amplifier 513, and the
amplified voltage is applied to an ultrasonic standard sound source (hereinafter referred to as a
standard sound source) 503 to propagate a predetermined ultrasonic wave 525 from the
standard sound source 503. .
[0037]
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13
The position of the ultrasonic microphone 100 is detected by the position sensor 511, and the
detection signal is transmitted to the sound field forming unit 516, and is related to the peak
level of each harmonic component at that position.
The peak levels of the respective harmonic components are converted into digital signals through
the digital oscilloscope 514, and are subjected to FFT processing by the fast Fourier transform
circuit in the harmonic separator 515, and the fundamental wave component, the second
harmonic component and the third harmonic The maximum value of the component is read, and
the read value (unit: volt (V)) is output to the sound field former 516 (fifth step, sixth step).
[0038]
Then, in the sound field formation unit 516, the position information (measured position)
detected by the position sensor 511 is associated with the output value from the harmonic
separator 515. In this way, sound field data acquisition on the basis of actual measurement of
volts (V) is performed. This data is stored in the memory of the reception sensitivity calculation
means 521.
[0039]
On the other hand, with respect to the fundamental wave, the second harmonic wave, and the
third harmonic wave, based on the characteristics of the standard sound source 503 (for
example, information such as dimensions of the standard sound source, sound source sound
pressure, frequency, etc.) The integral equation, the fundamental wave, the second harmonic
wave and the third harmonic wave are calculated by the microcomputer 519 using the KZK nonlinear differential equation (third step), theoretical sound field data of the unit Pa (calculated
sound Pressure) can be obtained. This data is stored in the memory of the reception sensitivity
calculation means 521. Then, with respect to the sound field calculated in the third step, a table
indicating the relationship between the sound pressure and the position is created (fourth step).
[0040]
Then, the reception sensitivity computing means 521 obtains the reception sensitivity of the unit
V / Pa from the actually measured sound field data of the unit volt (V) and the theoretical
04-05-2019
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calculated sound field data of the unit Pa (the eighth step) ).
[0041]
The sensitivity measurement procedure will be described in detail with reference to FIGS. 6 and
7.
First, a wide-band ultrasonic piston sound source (standard sound source) 503 (see FIG. 6)
having an aperture radius a and capable of transmitting an ultrasonic wave of center frequency f
0 is prepared (see reference numeral 401: first step) . The aperture radius a is preferably set to a
size that varies depending on the frequency. For example, a Langevan vibrator made of a
piezoelectric ceramic material of 40 mm in diameter is used because 20 mm in water for
underwater use and the frequency is low in air.
[0042]
The vibration surface of the Langevin vibrator can excite thickness longitudinal vibration in a
single vibration mode over the entire opening, which is preferable as a standard sound source for
ultrasonic pistons, but excites thickness longitudinal vibration in a single vibration mode over the
entire opening If it can be, it may be a normal thickness longitudinal vibrator.
[0043]
Then, a sine continuous wave of 50 to 100 Vpp is applied to the standard sound source 503
shown in FIG. 6 to transmit an ultrasonic wave of the center frequency f 0 (see reference numeral
403).
Furthermore, the vibration velocity u 0 of the surface of the standard sound source 503 is
measured by a laser Doppler. The sound source sound pressure P 0 is obtained by integrating the
acoustic impedance of the ultrasonic wave propagation medium with the vibration velocity u 0
(see reference numeral 404: second step). The relationship between the sound pressure in (Pa)
units and the sound pressure level (SPL) represented by (dB) described in FIG. 8 and FIG. 9 is SPL
= 20 LOG (P 1 / P 0), P 0: Reference Sound pressure (= 2 × 10 <−5> (Pa)).
04-05-2019
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[0044]
Next, the standard sound source 503 and the ultrasonic microphone 100 scheduled to be
calibrated are set in the ultrasonic microphone sensitivity measuring device 500 shown in FIG. 6,
and the fundamental wave component ffund = f while the ultrasonic microphone 100 is moved
by the manipulator 507. Measure (scan) the sound pressure at the observation points (xi, yj, zk)
of 0, the second harmonic component f 2nd = 2 f 0, and the third harmonic component f 3 rd = 3
f 0 (see reference numeral 405: fifth Step). i, j, k are the measured points along the respective
axes x, y, z respectively.
[0045]
It is not necessary to measure all the observation points along all the axes, for example, by
changing k in the z-axis direction, that is, the ultrasonic wave propagation direction, or i or x
about the x or y axis perpendicular to the z axis. Change j and measure. The unit of the
measurement amount in this case is the voltage (output voltage V meas) (V).
[0046]
The output of the signal generator 512 is amplified by the power amplifier 513, and this
amplified voltage is applied to the standard sound source 503 and transmitted from the standard
sound source 503. The ultrasonic microphone 100 at a predetermined position (xi, yj, zk)
receives the ultrasonic wave transmitted from the standard sound source 503, and converts the
ultrasonic sound pressure into a voltage signal (seventh step). The ratio of the output voltage to
the received sound pressure P cal at this time is defined as the reception sensitivity and is
expressed in units of (V / Pa). The voltage signal output from the ultrasonic microphone 100 is a
time axis signal, and since each harmonic component is superimposed, in order to extract a
voltage for each harmonic component, the time axis signal is subjected to FFT processing to
obtain a basic signal. Processing is performed by the harmonic separator 515 to determine peak
levels for each of the wave component, the second harmonic component, and the third harmonic
component (ie, for each frequency component) (see reference numeral 406: sixth step).
[0047]
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Then, in the sound field formation unit 516, the position information detected by the position
sensor 511 is associated with the output value of the peak level from the harmonic separator
515. In this way, measured sound field data acquisition of the voltage (unit is volt (V)) of the
fundamental wave component, the second harmonic component and the third harmonic
component is performed. The sound field data is stored in the memory of the reception
sensitivity calculation unit 521.
[0048]
Although harmonic components exist at the fourth harmonic or higher, the sound pressure level
decreases as the higher harmonics are reached, and the measurement error increases. Therefore,
high-frequency components at the fourth harmonic or higher are used for reception sensitivity.
Care should be taken when determining (V / Pa).
[0049]
By the above method, unit conversion can be performed at three frequencies of the fundamental
wave frequency, the second harmonic frequency, and the third harmonic wave. However, in order
to perform unit conversion at a larger number of frequency points, Using a wide band transducer
such as a composite piezoelectric transducer as an acoustic wave source, the frequency of the
drive signal is set in small increments within ± 50% of the resonant frequency in the vicinity of
its resonant frequency, Extract harmonics and third harmonic components (eighth step)).
[0050]
For example, when a sound source with a center frequency of 200 kHz and a −3 dB frequency
band of 100 kHz to 300 kHz is used as a reference sound source, assuming that the drive
frequency pitch is 20 kHz, the fundamental frequency is 200 kHz ± 20 nkHz (n = 1 to 5 integer)
It can be set to
For these fundamental frequencies, the second harmonic frequency is represented by 2 × (200
kHz ± 20 nkHz), and the frequency of the third harmonic component is represented by 3 ×
(200 kHz ± 20 n).
[0051]
Therefore, 100 kHz, 120 kHz, 140 kHz, 160 kHz, 180 kHz, 200 kHz, 240 kHz, 260 kHz, 280
kHz, 300 kHz fundamental wave frequency, 200 kHz, 240 kHz, 280 kHz, 320 kHz, 320 kHz, 360
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kHz, 400 kHz second harmonic frequency, 300 kHz, 360 kHz, The third harmonic setting
frequency point of 420 kHz, 480 kHz, 540 kHz, 600 kHz, 660 kHz, 720 kHz, 780 kHz, 840 kHz,
and 900 kHz is possible.
[0052]
As described above, when the horizontal axis is frequency, the vertical axis is [V / Pa], and
ultrasonic waves with a sound pressure of 1 μPa are received, the hydrophone is a unit in which
the time when the voltage of 1 V can be obtained is 0 dB. It is possible to use (dB re V / μPa)
used for the calibration data of
In addition, it may be used for the calibration data of a hydrophone (dB re V / Pa) in the unit
when 0V is obtained when the voltage of 1V is obtained when the ultrasonic wave of the sound
pressure of 1Pa is received. .
[0053]
The reception sensitivity calculation means 521 calculates theoretical sound field data.
First, a virtual ultrasonic piston reference sound source having an aperture radius a and capable
of transmitting an ultrasonic wave at the center frequency f0 is assumed (see reference numeral
402).
[0054]
By using the sound source sound pressure P 0 obtained by integrating the acoustic impedance of
the ultrasonic wave propagation medium to the vibration velocity u 0 of the surface of the
standard sound source 503, the fundamental wave component is obtained by the KZK nonlinear
differential equation (equation 1) Calculate the sound pressure at the observation point (xi, yj, zk)
of ffund = f0, second harmonic component f2nd = 2f0, third harmonic component f3rd = 3f0 (see
reference numeral 409: third step) ). i, j, k are the measured points along the respective axes x, y,
z respectively. Moreover, the unit of the sound pressure obtained here is Pa.
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[0055]
<img class = "EMIRef" id = "446393787-000003" /> For details regarding this equation 1, see:
The Basics of Nonlinear Acoustics by Tomoo Kasakura (Aichi Publishing 1996, Chapter 5).
[0056]
Thereafter, the peak sound pressure (unit: Pa) of the sound pressure data at the observation
point (xi, yj, zk) is extracted with respect to the fundamental wave component, the second
harmonic component, and the third harmonic component (see symbol 409).
Then, the extracted sound field data (see reference numeral 410) of the peak sound pressure is
stored in the memory of the reception sensitivity calculation means 521.
[0057]
In the reception sensitivity calculation means 521, the peak voltage (unit: V) of the voltage data
at the actually measured observation point (xi, yj, zk) with respect to the fundamental wave
component, the second harmonic component and the third harmonic component is The sound
pressure conversion reception sensitivity (V / Pa) can be obtained by dividing the sound pressure
data at the upper observation point (xi, yj, zk) by the peak sound pressure (unit: Pa).
[0058]
Then, the first sound pressure conversion receiving sensitivity (V / Pa) obtained by the
fundamental wave component, the second sound pressure conversion receiving sensitivity (V /
Pa) obtained by the second harmonic component, the third The agreement (in other words, the
difference) between the actual measurement value and the calculated value of each of the third
reception sensitivity for sound pressure conversion (V / Pa) obtained by the harmonic
component is evaluated, and the agreement is within 5%. If there is, the average value of the first
sound pressure conversion reception sensitivity, the second sound pressure conversion reception
sensitivity, and the third sound pressure conversion reception sensitivity is set as the normal
sound pressure conversion reception sensitivity (V / Pa). , And stored in the memory of the
ultrasonic wave received sound pressure converter 128 of the ultrasonic microphone device 1.
[0059]
In practice, in order to obtain a normal sound pressure conversion reception sensitivity (V / Pa),
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measurement is repeated using a standard sound source 503 having transducers having different
center frequencies, and the average value thereof is used for sound pressure conversion. The
reception sensitivity (V / Pa) is used.
[0060]
In the ultrasonic wave reception sound pressure conversion unit 128 of the ultrasonic
microphone device 1, the output voltage V meas fund (V) of the ultrasonic microphone 100 (in
the case of the fundamental wave), V meas 2nd (V) (in the case of the second harmonic
component) , V meas 3rd (V) (in the case of the third harmonic component) with the stored
reception sensitivity S rec (V / Pa) and divide P meas (V) by S rec (V / Pa) Received sound
pressure P cal fund (Pa) (for fundamental wave component), P cal 2nd (Pa) (for second harmonic
component), P cal 3rd (Pa) (for third harmonic component) Calculate
The calculation result is displayed by a display (not shown).
[0061]
[Approximation of Measured Voltage (V) by Ultrasonic Microphone 100 and Sound Pressure (Pa)
by Calculation] FIG. 8 shows a fundamental wave component, a second harmonic, a sound field
by a virtual standard sound source in water. Voltage obtained by measuring the sound field by
the ultrasonic microphone 100 using, as a sound source, a curve (unit dB) in which the third
harmonic component is calculated and the ultrasonic source having the standard sound source
and ultrasonic transmission characteristics used for the calculation as a sound source It is the
figure which plotted (V) and for every predetermined position by the same observation distance.
[0062]
Further, FIG. 9 is a diagram in which calculation and measurement are performed in the air as in
FIG. 8 and plotted at the same observation distance at predetermined positions.
“●” “+” “+”: actual measurement result, solid line: theoretical curve calculated from the
equation of KZK, dotted line: theoretical curve calculated from the equation of SBE (spheroidal
beam equation).
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Although FIG. 8 and FIG. 9 represent the calculation result in dB, the unit V of the measured
voltage value is not described.
[0063]
Although the actual measurement value is in units of V, as shown in FIGS. 8 and 9, it can be seen
that there is an approximation between the calculated sound field pattern and the measured
sound field pattern.
In particular, in FIG. 8, calculated sound field patterns for the main peak 704 (fundamental wave
1.69 MHz), the main peak 706 (second harmonic 3.2 MHz), and the main peak 707 (third
harmonic 4.8 MHz) The measured sound field pattern and the measured sound field pattern have
high similarity but different units, and can be regarded as identical. From such a point of view,
the reception sensitivity of the ultrasonic microphone 100 can be expressed in V / Pa units by
taking the ratio between the voltage obtained by measurement and the sound pressure obtained
by theory.
[0064]
The aerial sound field pattern shown in FIG. 9 can be considered similarly to the case of the
aerial sound field pattern. In the case of the air, the fundamental wave source is 30 kHz, the
second harmonic wave 60 kHz, and the third harmonic wave 90 kHz as a standard sound source,
and the correspondence between the calculation result and the actual value is made And the
third harmonic (90 kHz) are plotted.
[0065]
Also in this case, similarly to the result shown in FIG. 8, with respect to the main peak 704
(fundamental wave 30 kHz), the main peak 706 (second harmonic 60 kHz), and the main peak
707 (third harmonic 90 kHz), The measured sound field pattern is different in unit but has high
approximation and can be regarded as identical.
[0066]
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As described above, in the present invention, it is determined that the actual measurement value
and the calculation value are different characteristics but different units but the actual
measurement value P meas (V) is equal to the calculation value P cal (Pa), that is, the reception
sensitivity S rec (V / Pa) can be converted to S rec (V / Pa) = P meas (V) / P cal (Pa).
That is, the present invention can be easily converted into an acoustic unit, a bolt, into a sound
unit, Pascal or decibel by a relatively simple device, and is applied to an ultrasonic microphone
100 capable of easily receiving harmonics in the air. It has the effect of being able to
[0067]
It goes without saying that the present invention is not limited to the embodiments described
above. As shown in FIG. 10A, an ultrasonic microphone apparatus 200 according to another
modification includes an ultrasonic microphone main body 201 in which only the ultrasonic
microphone 100 is accommodated in the housing 107, and the ultrasonic microphone 100
outside the housing 107. It is comprised by the connected impedance conversion part (not
shown), and the ultrasonic wave reception sound pressure conversion part (ultrasonic wave
reception sound pressure conversion means) not shown connected with the impedance
conversion part outside the housing 107. FIG.
[0068]
As shown in FIG. 10B, the ultrasonic microphone device 250 according to still another
modification houses the ultrasonic microphone 100 and an impedance conversion unit (not
shown) connected to the ultrasonic microphone 100 in the housing 107. The ultrasonic
microphone main body 251 is composed of an ultrasonic sound pressure converter (not shown)
connected to the impedance converter 126 outside the housing 107 (ultrasonic sound pressure
converter).
[0069]
Moreover, although the unit was set to Pa as said sound pressure, you may utilize dB as a sound
pressure unit.
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When calculating the sound pressure, instead of the above-mentioned KZK non-linear differential
equation, Rayleigh's integral equation may be used for the fundamental wave. The sound
pressure conversion reception sensitivity may be in units of V / Pa or the reciprocal of V / dB.
[0070]
1, 250, 250: ultrasonic microphone device 100: ultrasonic microphone 101: piezoelectric electret
(acoustoelectric conversion element) 102, 103: electrode 107: housing 126: impedance
conversion unit 128: ultrasonic reception sound pressure conversion unit (ultrasound Received
sound pressure conversion means) ... ultrasonic standard sound source (standard sound source)
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