JP2017112582A - Ultrasonic microphone device and method for converting volt in electric unit into pascal or decibel in acoustic unit regarding output signal of ultrasonic microphone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ultrasonic microphone device that is easy to measure and allows sound pressure to be displayed in a high-frequency band, and a method for converting volt in an electric unit into Pascal or decibel in an acoustic unit regarding an output signal of an ultrasonic microphone.SOLUTION: In an ultrasonic microphone sensitivity measuring device 500 used in an ultrasonic frequency region, an ultrasonic microphone 100 for receiving an ultrasonic wave has sound pressure conversion reception sensitivity with a unit of V/Pa or V/dB, or a unit of their inverse and includes ultrasonic reception sound pressure conversion means for converting output voltage from the ultrasonic microphone 100 at an observation point into ultrasonic wave reception sound pressure on the basis of the reception sensitivity.SELECTED DRAWING: Figure 6

Description

  The present invention relates to an ultrasonic microphone device and a method for converting an output signal of an ultrasonic microphone from a voltage in electrical units to a pascal or decibel in acoustic units.

In recent years, technologies using ultrasonic signals have been widely used in the fields of medical diagnosis and non-destructive inspection, and have become established as basic technologies indispensable for the construction of a medical infrastructure or a safe and secure social infrastructure. Examples of the application of ultrasonic signals in the field of non-destructive inspection include the diagnosis of objects that cannot be destroyed due to inspection, such as the presence of cracks in infrastructure structures such as tunnels, expressways, nuclear reactors, and buildings. .
Although this ultrasonic technology does not have to worry about exposure like the X-ray technology, the possibility of ultrasonic exposure has been pointed out when using power ultrasonic applications using nonlinear ultrasonic waves or continuous waves. It is pointed out that if an ultrasonic wave of a certain intensity or higher is exposed to a person's ear, it may have some effect on the person.
Regarding the measurement of sound pressure, for audible sound, there is a sound level meter that can display the loudness at the measurement point in dB, and the objective physical property of the sound can be quantified. As for the ultrasonic wave, a hydrophone that is 100 kHz or less, or an object that is a living body and has an acoustic characteristic similar to water, and that can measure an observation point sound pressure in water is commercially available.
On the other hand, for example, what can be expressed as a sound pressure value for nonlinear acoustic waves called fractional harmonics contained in high-frequency aerial ultrasonic waves of several hundred kHz to several MHz, is a dedicated measuring instrument using a method called a balance method, A method using a mutual rehabilitation method is known.

JP 09-331599 A

However, when measuring an aerial ultrasonic wave using an apparatus using a balance method, there is a restriction on the observation point position, that is, the measurement point is a fixed point, that is, the position of the balance pan for the ultrasonic transmission sound source to be measured. In addition, this apparatus requires a windshield because the influence of the air flow greatly affects the measurement accuracy and gives a measurement error. However, such a restriction is not suitable for the measurement to cope with the ultrasonic exposure described above.
In the mutual regeneration method, three ultrasonic sensors are prepared and two of them are selected, and each of them is used for transmission or reception. In this method, the reception sensitivity is estimated and calibrated from the relationship between the voltage and the reception voltage at the time of reception. However, in the case of using the mutual rehabilitation method, it is necessary to make the opening dimension to a dimension equal to or smaller 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 have sufficient transmission sensitivity for transmission and sufficient reception sensitivity for reception, but there is no piezoelectric material that has a small aperture size and can satisfy the sensitivity conditions. .

  Therefore, the present invention provides an ultrasonic microphone device that can be easily measured and can display sound pressure in a high frequency band.

The ultrasonic microphone device of the present invention is an ultrasonic microphone device used in an ultrasonic frequency range, and an ultrasonic microphone that receives ultrasonic waves has a unit of V / Pa or V / dB, or a reciprocal thereof. It has a sound pressure conversion receiving unit for sound pressure conversion as a unit, and has an ultrasonic reception sound pressure conversion means for converting an output voltage from the ultrasonic microphone at an observation point into an ultrasonic reception sound pressure based on the reception sensitivity. It is characterized by that.
This ultrasonic microphone device can display the ultrasonic wave with a sound pressure of Pa or dB so that the user can visually recognize it, thereby improving the convenience for the user.

(2) The ultrasonic reception sound pressure conversion means of the present invention compares 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. Then, the reception sound pressure P _cal (Pa) may be calculated by dividing the output voltage V _meas (V) by the reception sensitivity S _rec .

(3) The reception sensitivity of the present invention may be created through the following steps:
First step: A broadband ultrasonic piston sound source having an opening radius a and generating and propagating a fundamental ultrasonic wave of a frequency f ₀ that generates a higher-order harmonic of a frequency nf ₀ (n is an integer) when propagating to the air An ultrasonic standard sound source will be provided.
Second step: Measure 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 this vibration velocity u ₀ .
Third step: Using the vibration velocity u ₀ or the sound source sound pressure P ₀ measured in the second step, the opening radius a, and the frequency f ₀ , and using the Rayleigh integral formula, the ultrasonic standard Calculate the sound pressure P _{cal fund} of the fundamental wave formed by the sound source in units of Pascal (Pa) or decibel (dB), or use the nonlinear differential equation of KZK, The sound pressure P _{cal fund} , the second harmonic component sound pressure P _{cal 2nd} and the third harmonic component sound pressure P _{cal # 3rd} are calculated in units of Pascal (Pa) or Decibel (dB).
Fourth step: For the sound field calculated in the third step, a table indicating the relationship between sound pressure and position is created.
Fifth step: The ultrasonic microphone is sequentially scanned at each position set in the table created in step 4, and the output voltage V _meas of the ultrasonic microphone at each position is measured.
Sixth step: The output voltage V _meas of the ultrasonic microphone actually measured in the fifth step is _expressed as a fundamental wave component V _{meas fund} , a second harmonic component V _{meas 2nd,} and a third harmonic component V _{meas 3rd} . And the voltage level value of each frequency component is extracted.
Seventh step: The sound pressure of the fundamental wave component, the sound pressure of the second harmonic component, and the sound pressure of the third harmonic component at each position calculated in the fourth step P _{cal fund} , P _{cal 2nd} , P _{cal 3rd} and the fundamental wave component, the second harmonic component, and the third harmonic component V _{meas fund} , V _{meas 2nd} , P _meas of the output voltage at the measured position of the fifth step corresponding to each position Based on _3rd , the respective reception sensitivities V _{meas fund} / P _{cal fund} , V _{meas 2nd} / P _{cal 2nd} and V _{meas 3rd} / P _{cal 3rd} are calculated.
Eighth step: Receiving sensitivity V _{meas fund} / P _{cal fund} , V _{meas 2nd} / P _{cal 2nd} , and V _{meas 3rd} / P _{cal 3rd} of the fundamental wave component, the second harmonic component, and the third harmonic component, respectively. The coincidence is evaluated, and an average value of the measured value and the calculated value of the reception sensitivity whose difference from the coincidence is within 5% is calculated, and this average value is set as the reception sensitivity of the ultrasonic microphone.
By operating the procedure from the first step to the eighth step, the theoretical sound field pattern of the relationship between the reference sound source specification (aperture radius, frequency, vibration speed on the ultrasonic transmission surface) and the relative coordinates between the observation points is obtained. Calculated in unit Pascal (Pa) or decibel (dB), and set so that the relationship between the reference sound source and the observation position is the same as in the theoretical calculation, and the measured sound in the unit of volts (V) By measuring the field and evaluating the coincidence (correlation coefficient) between the acoustic focal regions of both patterns, it is determined that the same physical quantity of both is observed, and the unit of the ultrasonic microphone is (V / Pa ) Or (V / dB). Then, the theoretical sound field and the actually measured sound field at a plurality of points in the acoustic focal region with the least error factor (that is, a plurality of points sandwiched between positions (two points) that are half the peak value). The coincidence is evaluated by the correlation coefficient between the calculated value and the actual measurement value at a plurality of positions sandwiched between positions (two points) that are ½ the peak value. Therefore, it is possible to provide an ultrasonic microphone with a reliable sensitivity display.
The KZK nonlinear differential equation means a (Khokhlov-Zabolotskaya-Kuznetsov) nonlinear differential equation. The same applies hereinafter.

(4) The ultrasonic microphone according to the present invention has a porous polypropylene film having gold formed on both surfaces as an acoustoelectric conversion element for converting received ultrasonic waves into an electric signal, and charges the porous polypropylene film. Alternatively, a piezoelectric electret may be used.
As the acoustoelectric transducer that is the heart of an ultrasonic microphone, a piezoelectric electret vibrator charged with a porous polypropylene film exhibiting broadband, high sensitivity and high SN (signal / noise ratio) characteristics is used. The ultrasonic sound field can be measured with high reliability.

(5) The ultrasonic microphone of the present invention may have a protective film layer.
In the present invention, since a weather-resistant protective film is formed on the ultrasonic wave receiving surface, the ultrasonic microphone is particularly resistant to the environment such as moisture proof and dust proof.

(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.
Built-in impedance conversion circuit of source follower type FET input and placed in the immediate vicinity of the piezoelectric electret vibrator, so it is not affected by the length of cable to be connected, intrinsic impedance, capacitance, and influence of flying noise This makes it possible to provide an ultrasonic microphone with a good SNR.

(7) The ultrasonic reception sound pressure conversion means connected to the impedance conversion unit of the present invention may be accommodated in the housing.
Since the functions related to all the steps described in claim 3 are integrated in the ultrasonic microphone housing, it is possible to directly provide the user with sound pressure in an acoustic unit.

(8) The method of converting the output signal of the ultrasonic microphone of the present invention from an electric unit of volt to an acoustic unit of Pascal or decibel has an opening radius a and a frequency nf ₀ (n is an integer) when propagating into 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 having a frequency f ₀ that generates a higher-order harmonic, and a vibration velocity u of the surface of the ultrasonic standard sound source ₀ or a second step of measuring the source sound pressure P ₀ obtained by integrating an acoustic impedance to the vibration velocity u _0, and the vibration velocity u ₀ or the source sound pressure P ₀ was measured in the second step, the opening radius using the a, and the frequency f _0, or the calculated in units of acoustic pressure P _{cal fund} Pascal fundamental wave ultrasound standard source forms (Pa) or decibels (dB) using an integrating type of Rayleigh Or by using a non-linear differential equations of KZK, the sound pressure P _{cal fund} of the fundamental wave ultrasound standard sound source is formed, the sound pressure P _{cal 3rd} of the sound pressure P _{cal 2nd} and third harmonic components of the second harmonic component A third step of calculating a unit of Pascal (Pa) or decibel (dB), and a fourth step of creating a table showing the relationship between sound pressure and position with respect to the sound field calculated in the third step; In the fifth step, the ultrasonic microphone is sequentially scanned at each position set in the table created in the step 4, and the output voltage V _meas of the ultrasonic microphone at each position is actually measured. The measured output voltage V _meas of the ultrasonic microphone 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} . Voltage level A sixth step of extracting values, a sound pressure of the fundamental component, a sound pressure of the second harmonic component, and a sound pressure P _{cal fund} of the third harmonic component at each position calculated in the fourth step. , P _{cal 2nd} , P _{cal 3rd} , the fundamental wave component, the second harmonic component, and the third harmonic component V _{meas fund} , V of the output voltage at the measured position of the fifth step corresponding to each position _{Based on meas 2nd} and V _{meas 3rd} , the seventh step of calculating the respective reception sensitivities V _{meas fund} / P _{cal fund} , V _{meas 2nd} / P _{cal 2nd} and V _{meas 3rd} / P _{cal 3rd} , fundamental wave component and When the coincidence of the reception sensitivities V _{meas fund} / P _{cal fund} , V _{meas 2nd} / P _{cal 2nd} , and V _{meas 3rd} / P _{cal 3rd} of the second harmonic component and the third harmonic component is matched, The average value between the measured value and the calculated value of the reception sensitivity with a difference of 5% or less is calculated, and this average value is calculated as the supersonic wave. And an eighth step for setting the reception sensitivity of the microwave microphone.

  The ultrasonic microphone device of the present invention is easy to measure and has the effect of enabling display of sound pressure in a high frequency band.

It is sectional drawing which shows one Embodiment of the ultrasonic microphone apparatus which concerns on this invention. It is a schematic diagram of the piezoelectric electret of the ultrasonic microphone device according to the present invention. It is the schematic diagram which showed the ultrasonic reception principle of the piezoelectric electret of the ultrasonic microphone apparatus which concerns on this invention. It is the schematic diagram which showed the ultrasonic reception principle of the piezoelectric electret of the ultrasonic microphone apparatus which concerns on this invention. It is a circuit diagram which shows the impedance conversion circuit of the ultrasonic microphone apparatus which concerns on this invention. It is a figure which shows the ultrasonic microphone sensitivity measuring apparatus of the ultrasonic microphone apparatus which concerns on this invention. It is the flowchart which showed the procedure for determining the receiving sensitivity of the ultrasonic microphone of the ultrasonic microphone apparatus which concerns on this invention. It is a graph which shows the coincidence of the underwater measured sound field and the calculated sound field. It is a graph which shows the coincidence of the sound field in the air and the calculated sound field. (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.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of an ultrasonic microphone device used in an ultrasonic frequency region (for example, several hundred kHz to several MHz or less) according to an embodiment of the present invention will be described in detail with reference to the drawings.

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 converter 126 connected to the ultrasonic microphone 100, and a signal processing circuit. And an ultrasonic reception sound pressure conversion unit (ultrasonic 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 housed in a conductive housing 107.

  The ultrasonic microphone 100 includes a piezoelectric electret 101 disposed in an opening 107 a on the front side of a housing 107, electrodes 102 and 103 provided so as to be in close contact with both surfaces of the piezoelectric electret 101, and an opening on the back side of the piezoelectric electret 101. The metal backing member 104 joined to the electrode 103 via the adhesive layer 106 at the portion 107b. The opening 107 a of the housing 107 is provided with a resin protective film layer 105 that is in close contact with the electrode 102, and the opening 107 b of the housing 107 is closed by a conductive cap 108.

An insulating layer 112 is formed on the inner wall surface of the housing 107 so that the electrode 102, the electrode 103, and the metal backing member 104 are not short-circuited.
The electrode 102 can be electrically connected to the housing 107 and the cap 108 through a conducting member 110 such as a lead wire, and is connected to the impedance converter 126 through the wiring 119b.

  The electrode 103 can be electrically connected to the backing member 104 through an insulating thin film adhesive layer 106, and is connected to the impedance converter 126 through the wiring 119a.

That is, in the present embodiment, the piezoelectric electret 101 uses the λ / 4 resonance mode and the response frequency band is ½ that of the λ / 2 resonance mode.
For this purpose, the back side of the piezoelectric electret 101 that is firmly fixed by the metal backing member 104 is not displaced, and the boundary surface between the surface of the backing member 104 and the piezoelectric electret 101 is a node of vibration, so that the piezoelectric The surface side of the electret 101 is set to have a maximum vibration amplitude Amax as being free of displacement. For this purpose, an epoxy adhesive which has a low viscosity before curing and can be bonded thinly and hardly after curing is selected for the adhesive layer 106. When an epoxy adhesive is employed, the adhesive layer 106 is DC-conductive by being pressure-cured, so the signal cable 119 may be soldered to the metal backing member 104.

FIG. 2 is a diagram schematically showing the internal cross-sectional structure of the piezoelectric electret 101. As shown in FIG. 2, the piezoelectric electret 101 is a member provided with a porous polypropylene film.
As shown in FIG. 2, fine flat voids 302 are uniformly distributed at a volume ratio of about 50% in the piezoelectric electret 101 sandwiched between the electrodes 102 and 103 (see FIG. 1). The piezoelectric electret 101 is positively and negatively charged so as to surround the flat void 302 by charging means such as corona discharge.

  Hereinafter, this single charging unit 320 will be referred to as a charging dipole for the sake of convenience, the acting force on the electric field will be referred to as a charging dipole moment for the sake of convenience, and the amount thereof will be expressed as μ, and μ = q · z (q : Charge amount, z: Short axis distance of flat void).

This charged dipole moment μ is accumulated by the number N of flat voids 302 in the entire piezoelectric electret 101, and becomes Nμ in the entire piezoelectric electret 101. This is referred to as charge polarization P for convenience.
This charged polarization P is expressed by P = Nμ = N · q · z (where z is the short axis distance of the flat void). The charge of the charged polarization P is not observed from the outside because it is bound and neutralized by bound charges localized near the surface of the piezoelectric electret 101 as in the case of a normal piezoelectric body. This is shown in FIG.

  In order to make the following description easy to understand, the charge amount of the total charge polarization P is set to +10 on the upper surface side 309 and −10 on the lower surface side 310. As long as there is no external change such as pressure, temperature, pressure, etc., in the electrodes 102 and 103, the negative bound charge −10 and the positive bound charge +10 are balanced and neutralized. No charge is observed on 103.

  FIG. 4 shows this state together with the flat void 302. When ultrasonic waves are incident in such a neutralized state and the upper surface side electrode 102 is compressed and deformed by δz (see reference numeral 315) as shown in FIG. 4, the charged polarization P is expressed by the equation P = N · q · z. Accordingly, ΔP = N · q · δz decreases.

  For example, if the charge amount of the charged polarization P is changed to +8 on the upper surface side 309 and −8 on the lower surface side 310 due to compressive deformation, the bound charge cannot immediately change its state. Therefore, the charge amount of −2 on the upper surface side 309 and the difference +2 on the lower surface side 310 is focused on the electrodes 102 and 103. When the electrostatic capacitance of the piezoelectric electret 101 is C, this outflow charge Q is detected as a voltage value with V = Q / C. Alternatively, it is detected by the charge amplifier in the state of charge and converted into a voltage signal. This is the output voltage of the ultrasonic microphone 100.

The bound charges described above are localized at the interface between the electrodes 102 and 103 and the piezoelectric electret 101. Therefore, the adhesion at the interface between the electrodes 102 and 103 and the piezoelectric electret 101 is important.
If wrinkles or warpage occurs in the piezoelectric electret 101 due to electrode formation, stable acoustoelectric conversion cannot be performed. Therefore, deformation such as warping that affects the adhesion of the electrodes 102 and 103 and 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 pretreatment of the surface of the piezoelectric electret 101 and optimization of sputtering conditions.

As shown in FIG. 1, the ultrasonic reception sound pressure conversion unit 128 is connected to an impedance conversion unit 126 configured by a circuit via a wiring 127 and converts the output voltage from the impedance conversion unit 126 into ultrasonic reception sound pressure. The signal processing circuit is provided.
Since 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 a three-peak frequency characteristic, the maximum value at each peak is A1, Assuming A2 and A3, A1 is the fundamental wave component of the received ultrasound, its peak frequency is f ₀ , A2 is the second harmonic component of the received ultrasound, its peak frequency is 2f ₀ , and A3 is the third of the received ultrasound. The peak frequency of the harmonic component is 3f ₀ .

These parameters of A1 @ f ₀ , A2 @ 2f ₀ , A3 @ 3f ₀ can be easily extracted by digital calculation. These values are compared with position information from a position sensor provided in the vicinity of the ultrasonic microphone, that is, coordinate information (xi, yj, zk), and position functions A1 (xi, yj, zk), A2 ( xi, yj, zk) and A3 (xi, yj, zk) are obtained by a sound field generator. The units of A1, A2, and A3 that are actually measured values are all (V).

The ultrasonic reception sound pressure conversion unit 128 has a built-in memory. Specifically, in the ultrasonic reception sound pressure conversion unit 128, a Rayleigh integral equation and / or a KZK nonlinear differential equation whose unit is represented by a sound pressure unit [Pa] or a sound pressure level unit [dB] is stored in advance. For sound pressure conversion obtained from the relationship between the calculated sound field data calculated using, the actually measured sound field data obtained from the ultrasonic microphone 100, and the calculated sound field data and the actually measured sound field data. Reception sensitivity data S1 (xi, yj, zk), S2 (xi, yj, zk), S3 (xi, yj, zk) of reception sensitivity (unit: V / Pa) are stored.
The Rayleigh integral formula can calculate the calculated sound field data of the fundamental wave, and the KZK nonlinear differential equation can calculate the calculated sound field data of the fundamental wave, the second harmonic, and the third harmonic. Can do.

  As shown in FIG. 5, the impedance converter 126 uses an FET (field effect transistor) 215 with a high input impedance in the first stage of the circuit. A signal cable 119 for receiving a signal of the piezoelectric electret 101 and a gate resistor 213 having an extremely high resistance of GΩ are connected to the gate terminal 212 of the FET 215. A drain terminal 216 is provided on the drain 118 of the FET 215. A 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.

  The piezoelectric electret 101 has a characteristic that the dielectric constant is extremely small. For this reason, the piezoelectric electret 101 has an extremely large impedance that can be expressed by 1 / 2πfC (f: operating frequency, C: capacitance of the piezoelectric electret 101), and cannot be electrically matched with the input impedance of a subsequent circuit such as an amplifier. For this reason, multiple reflection noises are placed on the input part of the FET 215. If the input impedance is low and the 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 is not practical. If the source follower type impedance converter 126 shown in FIG. 5 is used, impedance matching and output impedance can be converted to be low, so that electrical matching with subsequent circuits can be achieved, and amplification is not performed. There is an advantage that it can be widely used.

  Next, the measurement procedure and measurement apparatus for the sound pressure conversion reception sensitivity (unit: V / Pa) of the ultrasonic microphone 100 will be described below.

  As shown in FIG. 6, the ultrasonic microphone sensitivity measuring apparatus 500 includes a manipulator 507 that can move 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. A standard sound source 503 as a broadband ultrasonic piston sound source capable of transmitting the ultrasonic wave, and a position sensor 511 for detecting the position of the ultrasonic microphone 100 moved by the manipulator 507.

The ultrasonic microphone 100 is suspended from the manipulator 507 while being held by the holder 505 (first step).
The output of the signal generator 512 is amplified by a power amplifier 513, and this 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. .

  The position of the ultrasonic microphone 100 is detected by the position sensor 511, and the detection signal is transmitted to the sound field generator 516 and related to the peak level of each harmonic component at that position. The peak level of each harmonic component is converted into a digital signal through a digital oscilloscope 514, and FFT processing is performed by a fast Fourier transform circuit in a harmonic separator 515, so that a fundamental wave component, a second harmonic component, and a third harmonic wave are processed. The maximum value of the component is read, and the read value (unit: volts (V)) is output to the sound field generator 516 (fifth step, sixth step).

  Then, in the sound field generator 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, the actual sound field data of the unit of volts (V) is taken. This data is stored in the memory of the reception sensitivity calculation means 521.

On the other hand, with respect to the fundamental wave, the second harmonic, and the third harmonic, based on the characteristics of the standard sound source 503 (for example, information on the dimensions of the standard sound source, sound source sound pressure, frequency, etc.) The integration formula, fundamental wave, second harmonic, and third harmonic are calculated by the microcomputer 519 using the KZK nonlinear differential equation (third step), and the calculated sound field data in the unit Pa (calculated sound). Pressure). This data is stored in the memory of the reception sensitivity calculation means 521.
Then, a table indicating the relationship between the sound pressure and the position is created for the sound field calculated in the third step (fourth step).

  Then, the reception sensitivity calculation means 521 obtains the reception sensitivity of the unit of V / Pa from the actually measured sound field data of the unit of volt (V) and the theoretically calculated sound field data of the unit of Pa (eighth step). ).

The sensitivity measurement procedure will be described in detail with reference to FIGS.
First, a broadband ultrasonic piston sound source (standard sound source) 503 (see FIG. 6) having an opening radius a and capable of transmitting an ultrasonic wave having a center frequency f ₀ is prepared (see reference numeral 401: first step). . The opening radius a is preferably different in size depending on the frequency. For example, a Langevin vibrator made of a piezoelectric ceramic material having a diameter of 20 mm for underwater use and a low frequency for air use is used.

  The vibration surface of the Langevin vibrator can excite the thickness longitudinal vibration of the single vibration mode over the entire aperture, which is preferable as an ultrasonic piston standard sound source, but excites the thickness longitudinal vibration of the single vibration mode over the entire aperture. If possible, a normal thickness longitudinal vibrator may be used.

Next, 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 having a center frequency f ₀ (see reference numeral 403). Further, the vibration velocity u ₀ of the surface of the standard sound source 503 is measured with a laser Doppler. The sound source sound pressure P ₀ is obtained by accumulating the acoustic impedance of the ultrasonic propagation medium to the vibration velocity u ₀ (see reference numeral 404: second step).
The relationship between the sound pressure in units of (Pa) and the sound pressure level (SPL) represented by (dB) shown in FIGS.
SPL = 20 LOG (P ₁ / P ₀ ), P ₀ : Reference sound pressure (= 2 × 10 ⁻⁵ (Pa))
It is.

Next, the standard sound source 503 and the ultrasonic microphone 100 to be calibrated are set in the ultrasonic microphone sensitivity measuring apparatus 500 shown in FIG. 6, and the fundamental wave component f _fund = f while moving the ultrasonic microphone 100 by the manipulator 507. ₀ , measure (scan) the sound pressure at the observation point (xi, yj, zk) of the second harmonic component f _2nd = 2f ₀ and the third harmonic component f _3rd = 3f ₀ (see reference numeral 405: fifth) Step). i, j, and k are the number of measurement points along the respective axes x, y, and z.

The measurement does not need to be performed for all observation points along all the axes. For example, measurement is performed by changing k in the z-axis direction, that is, the ultrasonic wave propagation direction, or i or x or y-axis perpendicular to the z-axis. What is necessary is just to measure by changing j. The unit of measurement in this case is voltage (output voltage V _meas ) (V).

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 between the output voltage and the reception sound pressure P _cal at this time is defined as reception sensitivity and expressed in units of (V / Pa). Since the voltage signal output from the ultrasonic microphone 100 is a time axis signal and each harmonic component is superimposed, the time axis signal is subjected to FFT processing in order to extract a voltage for each harmonic component. In order to determine the peak level for each of the wave component, the second harmonic component, and the third harmonic component (that is, for each frequency component), processing is performed by the harmonic separator 515 (see reference numeral 406: sixth step).

  Then, in the sound field generator 516, the position information detected by the position sensor 511 is associated with the peak level output value from the harmonic separator 515. In this way, the sound field data on the actual measurement of the voltages (unit: volts (V)) of the fundamental wave component, the second harmonic component, and the third harmonic component are obtained. This sound field data is stored in the memory of the reception sensitivity calculation means 521.

  Although higher harmonic components exist than the fourth harmonic, the higher the higher harmonics, the lower the sound pressure level and the greater the measurement error. Care must be taken when calculating (V / Pa).

  With the above method, unit conversion was possible at three frequencies of the fundamental frequency, second harmonic frequency, and third harmonic, but in order to perform unit conversion at a larger number of frequency points, A broadband vibrator such as a composite piezoelectric vibrator is used as the sound source, and the fundamental frequency is set in small increments within the range of ± 50% of the resonance frequency around the resonance frequency. Harmonic and third harmonic components are extracted (eighth step)).

  For example, when a sound source having 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, when the driving frequency pitch is 20 kHz, the fundamental frequency is 200 kHz ± 20 nkHz (n = 1 to 5). It can be set to. With respect to these fundamental wave 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).

  Therefore, 100kHz, 120kHz, 140kHz, 160kHz, 180kHz, 200kHz, 220kHz, 240kHz, 260kHz, 280kHz, 300kHz fundamental frequency, 200kHz, 240kHz, 280kHz, 320kHz, 360kHz, 400kHz second harmonic frequency, 300kHz, 360kHz, The set frequency point of the third harmonic frequency of 420kHz, 480kHz, 540kHz, 600kHz, 660kHz, 720kHz, 780kHz, 840kHz, 900kHz becomes possible.

  As described above, the horizontal axis is the frequency, the vertical axis is [V / Pa], and the unit when the voltage of 1V is obtained when the ultrasonic wave with the sound pressure of 1 μPa is received is 0 dB. It is possible to use (dB re V / μPa) used for the calibration data. Note that the unit (dB re V / Pa) used for hydrophone calibration data may be used in units of 0 dB when a voltage of 1 V is obtained when an ultrasonic wave having a sound pressure of 1 Pa is received. .

The reception sensitivity calculation means 521 calculates theoretical sound field data.
First, a hypothetical ultrasonic piston reference sound source having an opening radius a and capable of transmitting an ultrasonic wave having a center frequency f0 is assumed (see reference numeral 402).

Using the sound source sound pressure P ₀ obtained by accumulating the acoustic impedance of the ultrasonic 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). ffund = f _0, the second harmonic component f2nd = 2f _0, the observation point of the third harmonic component _{f3rd = 3f 0 (xi, yj} , zk) is calculated by calculating the sound pressure at (code 409 see third step ). i, j, and k are the number of measurement points along the respective axes x, y, and z. The unit of the sound pressure obtained here is Pa.

Formula 1

For details on Equation 1, see the author: Tomoo Kamakura, Fundamentals of Nonlinear Acoustics (Aichi Publishing, 1996), Chapter 5.

  Thereafter, for the fundamental wave component, the second harmonic component, and the third harmonic component, the peak sound pressure (unit: Pa) of the sound pressure data at the observation point (xi, yj, zk) is extracted (see reference numeral 409). Then, the extracted sound field data of the peak sound pressure (see reference numeral 410) is stored in the memory of the reception sensitivity calculation means 521.

  The reception sensitivity calculation means 521 calculates the peak voltage (unit: V) of the voltage data at the observed observation points (xi, yj, zk) with respect to the fundamental wave component, the second harmonic component, and the third harmonic component. By dividing by the peak sound pressure (unit: Pa) of the sound pressure data at the upper observation points (xi, yj, zk), the sound pressure conversion reception sensitivity (V / Pa) is obtained.

  The first sound pressure conversion reception sensitivity (V / Pa) obtained from the fundamental wave component, the second sound pressure conversion reception sensitivity (V / Pa) obtained from the second harmonic component, and the third The coincidence (in other words, the difference) between the measured value and the calculated value of the third receiving sensitivity for sound pressure conversion (V / Pa) obtained with the harmonic component is evaluated, and the coincidence 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 defined as the normal sound pressure conversion reception sensitivity (V / Pa). These are stored in the memory of the ultrasonic reception sound pressure conversion unit 128 of the ultrasonic microphone device 1.

  In practice, in order to obtain a normal sound pressure conversion reception sensitivity (V / Pa), measurement is repeated with a standard sound source 503 having transducers having different center frequencies, and the average value is used for sound pressure conversion. The reception sensitivity (V / Pa) is assumed.

In the ultrasonic reception sound pressure conversion unit 128 of the ultrasonic microphone device 1, the output voltage V _{meas fund} (V) (in the case of the fundamental wave), V _{meas 2nd} (V) (in the case of the second harmonic component) of the ultrasonic microphone 100. ), V _{meas 3rd} (V) (in the case of the third harmonic component) is compared with the stored reception sensitivity S _rec (V / Pa), and P _meas (V) is divided by S _rec (V / Pa). Received sound pressure P _{cal fund} (Pa) (in case of fundamental wave component), P _{cal 2nd} (Pa) (in case of second harmonic component), P _{cal 3rd} (Pa) (in case of third harmonic component) Is calculated. The calculation result is displayed on a display (not shown).

[Approximation between the actually measured voltage (V) by the ultrasonic microphone 100 and the calculated sound pressure (Pa)]
FIG. 8 shows a curve (unit dB) in which the fundamental wave component, the second harmonic wave, and the third harmonic wave component are calculated for the sound field of a virtual standard sound source in water, and the standard sound source and super It is the figure which plotted the voltage (V) which measured the sound field with the ultrasonic microphone 100 for every predetermined position with the same observation distance, using the ultrasonic transducer which has a sound wave transmission characteristic as a sound source.

  Further, FIG. 9 is a diagram in which calculation and measurement are performed in the air in the same manner as in FIG. 8 and plotted for each predetermined position at the same observation distance. “●”, “+”, “◯”: 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). 8 and 9 show the calculation result in the unit dB, but the unit V of the actually measured voltage value is not described.

  Although the unit of the actually measured value is 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 actually measured sound field pattern. In particular, in FIG. 8, the calculated sound field pattern 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 different units but have high approximation and can be regarded as the same. From this point of view, the reception sensitivity of the ultrasonic microphone 100 can be expressed in units of V / Pa by taking the ratio of the voltage obtained by actual measurement and the sound pressure obtained by theory.

  The aerial sound field pattern shown in FIG. 9 can be considered in the same manner as the aerial sound field pattern. In the case of the air, the basic sound source is 30 kHz, the second harmonic is 60 kHz, and the third harmonic is 90 kHz. Correspondence between the calculation result and the measured value is performed, and the fundamental wave (30 kHz) and the second harmonic (60 kHz ) And the third harmonic (90 kHz).

  In this case, similarly to the result shown in FIG. 8, the calculated sound field pattern and the main peak 704 (basic 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 the same.

As described above, in the present invention, the actually measured value and the calculated value are determined to have the same characteristics but have different units, and it is assumed that the actually measured value P _meas (V) is equal to the calculated value P _cal (Pa). _rec (V / Pa) can be converted to S _rec (V / Pa) = P _meas (V) / P _cal (Pa). That is, the present invention is applicable to the ultrasonic microphone 100 that can easily convert a bolt, which is an electrical unit, into a Pascal or a decibel, which is an acoustic unit, with a relatively simple device and can easily receive harmonics in the air. There is an effect that can be done.

It goes without saying that the present invention is not limited to the embodiment described above.
As shown in FIG. 10A, an ultrasonic microphone device 200 according to another modification includes an ultrasonic microphone main body 201 in which only the ultrasonic microphone 100 is accommodated in a housing 107, and the ultrasonic microphone 100 outside the housing 107. A wired impedance conversion unit (not shown) and an ultrasonic reception sound pressure conversion unit (ultrasonic reception sound pressure conversion means) (not shown) connected to the impedance conversion unit outside the housing 107 are configured.

As shown in FIG. 10B, the ultrasonic microphone device 250 of still another modified example accommodates an ultrasonic microphone 100 and an impedance converter (not shown) connected to the ultrasonic microphone 100 in a housing 107. The ultrasonic microphone main body 251 and an ultrasonic reception sound pressure conversion unit (ultrasonic reception sound pressure conversion means) (not shown) connected to the impedance conversion unit 126 outside the housing 107 are configured.

Moreover, although the unit is Pa as the sound pressure, dB may be used as the sound pressure unit.
When calculating the sound pressure, the Rayleigh integral formula may be used for the fundamental wave instead of the KZK nonlinear differential equation.
The reception sensitivity for sound pressure conversion may be based on the inverse of V / Pa or V / dB.

DESCRIPTION OF SYMBOLS 1,250,250 ... Ultrasonic microphone apparatus 100 ... Ultrasonic microphone 101 ... Piezoelectric electret (acoustoelectric conversion element)
102, 103 ... electrode 107 ... housing 126 ... impedance converter 128 ... ultrasonic wave reception sound pressure converter (ultrasonic wave reception sound pressure conversion means)
503 ... Ultrasonic standard sound source (standard sound source)

Claims (8)

An ultrasonic microphone device used in an ultrasonic frequency domain,
An ultrasonic microphone that receives ultrasonic waves has a sound pressure conversion reception sensitivity in units of V / Pa or V / dB, or a reciprocal thereof, and based on the reception sensitivity, An ultrasonic microphone device comprising ultrasonic reception sound pressure conversion means for converting an output voltage from an ultrasonic microphone into ultrasonic reception sound pressure. The ultrasonic reception sound pressure conversion means compares 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, and outputs the output voltage V _{2. The} ultrasonic microphone device according to claim 1, wherein the reception sound pressure P _cal (Pa) is calculated by dividing _meas (V) by the reception sensitivity S _rec . The ultrasonic microphone device according to claim 1, wherein the reception sensitivity is created through the following steps.
First step: A broadband ultrasonic piston sound source having an opening radius a and generating and propagating a fundamental ultrasonic wave of a frequency f ₀ that generates a higher-order harmonic of a frequency nf ₀ (n is an integer) when propagating to the air An ultrasonic standard sound source will be provided.
Second step: Measure 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 this vibration velocity u ₀ .
Third step: Using the vibration velocity u ₀ or the sound source sound pressure P ₀ measured in the second step, the opening radius a, and the frequency f ₀ , and using the Rayleigh integral formula, the ultrasonic standard Calculate the sound pressure P _{cal fund} of the fundamental wave formed by the sound source in units of Pascal (Pa) or decibel (dB), or use the nonlinear differential equation of KZK, The sound pressure P _{cal fund} , the second harmonic component sound pressure P _{cal 2nd} and the third harmonic component sound pressure P _{cal 3rd} are calculated in units of Pascal (Pa) or Decibel (dB).
Fourth step: For the sound field calculated in the third step, a table indicating the relationship between sound pressure and position is created.
Fifth step: The ultrasonic microphone is sequentially scanned at each position set in the table created in step 4, and the output voltage V _meas of the ultrasonic microphone at each position is measured.
Sixth step: The output voltage V _meas of the ultrasonic microphone actually measured in the fifth step is _expressed as a fundamental wave component V _{meas fund} , a second harmonic component V _{meas 2nd,} and a third harmonic component V _{meas 3rd} . And the voltage level value of each frequency component is extracted.
Seventh step: The sound pressure of the fundamental wave component, the sound pressure of the second harmonic component, and the sound pressure of the third harmonic component at each position calculated in the fourth step P _{cal fund} , P _{cal 2nd} , P _{cal 3rd} and the fundamental wave component, the second harmonic component, and the third harmonic component V _{meas fund} , V _{meas 2nd} , V _meas of the output voltage at the measurement position of the fifth step corresponding to each position Based on _3rd , the respective reception sensitivities V _{meas fund} / P _{cal fund} , V _{meas 2nd} / P _{cal 2nd} and V _{meas 3rd} / P _{cal 3rd} are calculated.
Eighth step: Receiving sensitivity V _{meas fund} / P _{cal fund} , V _{meas 2nd} / P _{cal 2nd} , and V _{meas 3rd} / P _{cal 3rd} of the fundamental wave component, the second harmonic component, and the third harmonic component, respectively. The coincidence is evaluated, and an average value of the measured value and the calculated value of the reception sensitivity whose difference from the coincidence is within 5% is calculated, and this average value is set as the reception sensitivity of the ultrasonic microphone.   The ultrasonic microphone uses a piezoelectric electret having a porous polypropylene film in which gold is formed on both surfaces as an acoustoelectric conversion element for converting received ultrasonic waves into an electric signal, and charging the porous polypropylene film. The ultrasonic microphone device according to any one of claims 1 to 3, wherein   The ultrasonic microphone device according to claim 1, wherein the ultrasonic microphone has a protective film layer.   The ultrasonic wave according to claim 1, wherein the ultrasonic microphone and a source follower type impedance converter connected to the ultrasonic microphone are accommodated in a housing. Microphone device.   The ultrasonic microphone device according to claim 6, wherein the ultrasonic reception sound pressure conversion unit connected to the impedance conversion unit is accommodated in the housing. Ultrasound that has an opening radius a and serves as a broadband ultrasonic piston sound source that generates and propagates fundamental wave ultrasonic waves of frequency f ₀ that generate high-order harmonics of frequency nf ₀ (n is an integer) when propagating into the air. A first step of providing a standard sound source;
A second step of measuring a vibration velocity u ₀ of the surface of the ultrasonic standard sound source or a sound source sound pressure P ₀ obtained by integrating an acoustic impedance to the vibration velocity u ₀ ;
Using the vibration velocity u ₀ or the sound source sound pressure P ₀ measured in the second step, the opening radius a, and the frequency f ₀ , the ultrasonic standard sound source is formed using Rayleigh's integral equation. The fundamental sound pressure P _{cal fund} is calculated by the unit of Pascal (Pa) or decibel (dB), or the fundamental sound pressure P _{cal generated} by the ultrasonic standard sound source using the KZK nonlinear differential equation. _fund , the third step of calculating the sound pressure P _{cal 2nd} of the second harmonic component and the sound pressure P _{cal 3rd} of the third harmonic component in units of pascal (Pa) or decibel (dB);
A fourth step of creating a table showing the relationship between sound pressure and position with respect to the sound field calculated in the third step;
A fifth step of measuring the output voltage V _meas of the ultrasonic microphone for each position by sequentially scanning the ultrasonic microphone at each position set in the table created in Step 4;
The output voltage V _meas of the ultrasonic microphone measured in the fifth step is separated into a fundamental component V _{meas fund} , a second harmonic component V _{meas 2nd,} and a third harmonic component V _{meas 3rd} , A sixth step of extracting the voltage level value of each frequency component;
The sound pressure of the fundamental wave component, the sound pressure of the second harmonic component, and the sound pressure of the third harmonic component at each position calculated in the fourth step, P _{cal fund} , P _{cal 2nd} , P _{cal 3rd} , , Based on the fundamental wave component, the second harmonic component, and the third harmonic component V _{meas fund} , V _{meas 2nd} , V _{meas 3rd} of the output voltage at the actually measured position in the fifth step corresponding to each position. A seventh step of calculating respective reception sensitivities V _{meas fund} / P _{cal fund} , V _{meas 2nd} / P _{cal 2nd} and V _{meas 3rd} / P _{cal 3rd} ;
Evaluate the coincidence of the reception sensitivities V _{meas fund} / P _{cal fund} , V _{meas 2nd} / P _{cal 2nd} , and V _{meas 3rd} / P _{cal 3rd} of the fundamental wave component, second harmonic component, and third harmonic component Then, an average value of the measured value and the calculated value of the reception sensitivity whose difference from the coincidence is within 5% is calculated, and the eighth step is used as the reception sensitivity of the ultrasonic microphone. A method of converting an output signal of an ultrasonic microphone having a voltage from an electrical unit of volt to an acoustic unit of pascal or decibel.