KR100638836B1 - An automatic calibaration system for a sound level meter and the method thereof - Google Patents

Abstract

An automatic calibration system and method for a noise level meter are provided to improve the arrival to a desired value by installing a measuring unit including an amplifier and a voltmeter. An automatic calibration system for a noise level meter comprises a driving unit including a signal generator generating a sine signal applied to a standard sound source and a driving unit digital voltmeter counting the frequency of the sine signal and measuring a voltage; and a measuring unit containing a measuring amplifier amplifying the output signal of a standard microphone and a noise level meter and a digital voltmeter measuring the output voltage of the amplifier.

Description

An automatic calibaration system for a sound level meter and the method

1 is a frequency correction curve used in a frequency correction circuit.

2 is a block diagram showing the operation principle of the sound level meter.

3 is a conceptual diagram illustrating a calibration principle of a sound level meter.

4 is a block diagram showing an example of an acoustic calibration system according to the present invention.

5 is a diagram illustrating functional units of an acoustic calibration system according to the present invention.

6 is an example of the result of the frequency response calibration of the precision sound level meter according to the present invention.

Fig. 7 is a graph showing the sound level output voltage and sound pressure level measured according to the present invention as a function of frequency according to the speaker input voltage.

FIG. 8 is a graph representing sound level output voltage and sound pressure level measured in the low frequency region as a function of speaker input voltage.

9 is a graph representing the sound level output voltage and sound pressure level measured in the high frequency region as a function of the speaker input voltage.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic calibration system for a sound level meter and a method thereof, and more particularly, to a system and a method for controlling the calibration using a linearity characteristic of a calibration system.

Sound pressure is the pressure of the air vibration that produces the sound, that is, the physical pressure indicated by the sound, and the sound pressure level (SPL) represents the magnitude of the measured sound pressure in decibels (dB) relative to the reference sound pressure. . Since the sound pressure range detected by humans is very large, such as 20 μPa to 200 Pa, the sound pressure level is measured in decibel units (dB) using the logarithm of the sound pressure amount to effectively display such a wide range of change. Sound pressure level SPL is defined by the following equation.

SPL = 10 log (p² / p _ref ²) = 10 log (p / p _ref ) ² = 20 log (p / p _ref )

Where SPL = sound pressure level (dB), p = sound pressure (Pa), and p _ref = reference sound pressure (2 × 10 ⁻⁵ Pa).

The reference sound pressure (p _ref ) is defined as the approximate threshold of hearing at 1,000 Hz, which is the smallest sound a human can hear in an environment without other sources, which is 2x10 ^-4 dyne internationally. / cm ² (= 2x10 ^-5 N / m² = 2x10 ^-5 Pa). Sound Pressure Level (SPL) is basically how loud the sound is.

SPL is distinguished from sound level, which is a subjective measure that compensates for changes in hearing sensitivity depending on frequency. SPL measures the magnitude of physical pressure caused by sound, and Sound level (SL) measures sound in frequency-correction. It means to measure subjectively.

The frequency (or hearing) correction circuit is a circuit that corrects the sensitivity of a person according to the frequency according to the physical quantity of sound. The A, B, and C corrections correspond to the 40, 70, and 100 phon curves of Fletcher-Munson's isometric The frequency characteristic of this correction circuit is shown in FIG. The most commonly used curve for evaluating noise such as construction noise and environmental noise is the A-calibration curve.

Since human ears have low sensitivity to low frequencies (especially below 250 Hz) and high frequencies (above 8 kHz), when measuring the subjective characteristics of sound pressure, these ear characteristics should be adjusted. This subjective sound pressure, which is measured weighted, is called the "sound level", and the objective measurement of the unweighted sound pressure is called the "sound pressure level" (SPL). do. Sound level is measured by a sound level meter (SLM).

In the configuration of the sound level meter, the pressure change of the sound wave is converted into an electrical signal by the microphone as shown in FIG. This electrical signal is amplified by the preamplifier and the main amplifier and then input to the auditory correction (A, B, C) circuits. The output signal of the hearing (or frequency) correction circuit is input to the rectifier circuit via an amplifier. The rectifier circuit converts an AC signal into a DC signal. Therefore, the AC signal input to the rectifier circuit is converted into an effective value and output. The output signal of the rectifier circuit is represented by the decibel value at the indicator through a slow, fast, impulse, and peak hold circuit that determines how fast the indicator value changes.

Sound pressure level (SPL) measurements are generally affected by the performance of all components, such as the microphones, amplifiers, indicators, and electronic network peripherals that make up the sound level meter. The calibration of the sound level meter is carried out in an anechoic room with the whole as a unit instead of component-specific measurements at the frequencies given in the relevant criteria.

Conventionally, in order to calibrate the sound pressure level using a sound level meter, an automatic calibration system of a precision SLM was developed using a computer centering on the hardware configuration of the controller, and the accuracy of ± 3dB was shown in 15 minutes of calibration time. In addition, an automated system using the RS-232-C interface has been proposed for the sound level meter calibration, and a search algorithm is used to find numerical solutions of unknown system characteristics such as the relationship of SPL to speaker input voltage. However, without the RS-232-C interface, the sound level meter (SLM) cannot be calibrated, and the linear relationship of loudspeaker input voltage to sound pressure is not taken into account. Therefore, SPL is expressed as the logarithm of the output voltage of the measurement system. By the way, there is a problem that the search operation by SPL comparison is not substantially the same as that by sound pressure comparison. In addition, a method of measuring the electrical input / output characteristics of the SLM using a digital voltmeter (DVM) instead of acoustic input / output has been proposed. This calibration method is useful in the absence of an anechoic system, but it does not adequately reflect the linear characteristics of the SLM in the sound field.

Accordingly, the present invention is to solve the above problems, using an algorithm for automatically calibrating the sound level meter in the anechoic chamber based on the linearity of the input voltage of the standard sound source and the output sound pressure of the standard microphone, accurate and fast sound level meter It is an object of the present invention to provide a calibration system and a control method for calibrating a sound level meter accurately and effectively by achieving a calibration condition of.

The present invention for achieving the above object, the drive unit including a signal generator for generating a sine signal applied to the standard sound source and a drive unit digital voltmeter for counting the frequency of the sine signal applied to the standard sound source and measuring the voltage And a measuring unit including a measuring amplifier for amplifying an output signal of a standard microphone or a sound level meter and a measuring unit digital voltmeter measuring the output voltage of the amplifier. (a) installing the standard microphone in a fixed position; (b) selecting an input voltage V ^s of a standard sound source by a signal generator; (c) applying a selective input voltage (V ^s ) to a standard sound source to apply a sound field to the standard microphone; (d) measuring the voltage (V ₂ ) generated by the standard microphone by the measuring unit digital voltmeter and recording the voltage value; (e) calculating and recording a sound pressure _pi corresponding to the voltage value recorded in step (d); (f) the target pressure if the tolerance within the range of the sound pressure (p _i) is the target pressure (p _t) recorded at the step (e) (p _t) input voltage (V ^SRC standard sound source are determined and corresponding to the obtained Recording); (g) If the target sound pressure is not obtained in step (f), the new selected input voltage V ^s _{i + 1} of the standard sound source is cyclic V ^s _{i + 1} = V ^s _i (p _t / p _i ). Calculating i (where i denotes the current repeat count and i + 1 indicates the next repeat count), and returning to step (c); To provide a sound level meter calibration method characterized in that to adjust the input voltage applied to the standard sound source, including.

In the step (g), if the repetition number (i) is 3 or more, the cyclic formula for calculating the selection input voltage V ^s _{i + 1} of the standard sound source is replaced by the cyclic formula based on the quadratic formula. It is to provide a sound level meter calibration method.

Hereinafter, exemplary embodiments will be described in detail with reference to the accompanying drawings. This embodiment allows for a better understanding of the objects, features and advantages of the present invention.

The sound level meter calibration is a method of measuring the indicated value of the sound level meter by comparing a sound level meter with a standard microphone using a relative comparison with the sound level meter to be calibrated, and the principle of the calibration is shown in FIG. Standard microphone is a microphone whose standard value is determined by the standard reversible calibration system given the ratio of the sound pressure applied to the microphone used for calibration and the ratio of the voltage generated by the microphone. Calibration of the sound level meter by the standard microphone is performed with the sound level meter to be calibrated in the same position X as the standard microphone, as shown in FIG. To determine the sound pressure p radiated by the sound source at the X position, first place a standard microphone of known sensitivity (S) at the X point and measure the open circuit voltage V ₀ of the microphone. The sound pressure applied to a standard microphone is then given by p = V ₀ / S. The sound level meter calibration is specified to use a sinusoidal signal of pure sound, the center frequency of the 1/3 octave band, with a reference sound pressure level of 85 dB. The corresponding sound pressure is 0.356 Pa. Therefore, the size of the electric sine signal applied to the input of the sound source (speaker system) is adjusted so that the sound pressure is 0.356 Pa. Record the voltage V ^s applied to the sound source at this time. This process is called sound field calibration, and it is repeated at the frequency to be calibrated to measure and record the voltage V ^s for each frequency.

Second, it reads an indication of the sound level meter P _m in a state where the drive sound source by sound source input voltage V ^s of the previously obtained were fixed calibration target sound level meter after removing the standard microphone at the same position. Find the difference between this reading and 85 dB and make sure it is within the tolerance range for each frequency. The reading of the sound level meter can be determined by reading it using a CC TV or by measuring the voltage of the sound output terminal of the sound level meter with a digital voltmeter. When using CC TV, it is difficult to fully automate. The method of measuring the output voltage of a sound level meter shall always be corrected by measuring the output sensitivity of the sound level meter given as the ratio of the output voltage to the sound level meter reading after the voltage measurement.

4 is a block diagram illustrating an automatic calibration system for a sound level meter as an example of an acoustic calibration system used in the present invention. Fig. 5 is an explanatory diagram showing this grouped by function. As shown in the figure, the SLM automatic calibration system includes an acoustic driving unit 100, an acoustic measuring unit 200, and a computer control unit 300.

The sound driver 100 includes a signal generator 101, a driver digital voltage meter (hereinafter referred to as DVM I) 103 having a frequency coefficient function, a power amplifier 102, and a speaker 104 that is a standard sound source. It is composed.

The acoustic measurement unit 200 includes a microphone 201 paired with a preamplifier, a measurement amplifier 202, a measurement unit digital voltmeter (DVM II) 203, and an oscilloscope 204.

The control unit 300 for controlling the entire system is preferably configured of a personal computer (PC).

The speaker 104 generating sound and the microphone 201 directly receiving the sound are installed at a set distance apart from the anechoic room. The anechoic chamber 10 is ceiling, floor, walls are all covered with sound absorbing material to minimize the reflection of the sound to prevent the occurrence of errors in the sound detection.

The signal generator 101 preferably uses a sine wave generator controlled by a computer. The power amplifier 102 is an amplifier that amplifies the signal output from the signal generator 101 and applies it to the speaker 104. Outputs an electrical signal input as a sound source as a sound wave.

The DVM I 103 is connected to the control unit 301 by GPIB to measure the speaker input voltage V ^s and measures the voltage applied to the speaker by the control unit 300.

The microphone 201 is spaced apart from the set distance (usually 1m distance) from the speaker 104 which is the sound source in the anechoic chamber 10, and detects the sound pressure output from the speaker 104 and outputs an electric signal. The electrical signal of the microphone 201 is amplified by the measuring amplifier 202 and applied to the oscilloscope 204 and the measuring unit digital voltmeter 203, and the voltage V ₂ is transmitted to the control computer 300. In the case of the standard microphone 201, a sound level is calibrated according to a standard mutual measurement method.

The oscilloscope 204 represents a change over time of the output signal of the microphone 201 amplified by the measurement amplifier 202 and the detection signal of the DVM I 101 which measured the voltage input to the speaker 104.

The controller 300 configured as a PC is preferably connected through an interface such as GPIB.

The DVM I 103 of the acoustic driving unit 100 and the DVM II 103 of the measuring unit 200 are connected to the control unit 300 to connect the speaker input voltage V ^s and the output voltage of the acoustic measuring unit 200 ( V ₂ ) is transmitted and the controller 300 determines the speaker input voltage using V ^s and V ₂ and calculates a corresponding sound pressure.

Looking at the process of calibrating the sound level meter by the algorithm used in the control unit 300 as follows.

The sound level to be calibrated is calibrated by comparing the measured sound pressure to the target sound pressure, such as the sound pressure measured by the standard microphone.

The calibration procedure of the sound level meter is as follows.

Step 1 Find and record the speaker input voltage V ^s . V ^s is the voltage outputting the target sound pressure p _t = 0.356 Pa (corresponding to 85 dB target SPL) at each frequency.

Step 2, the sound level meter (SLM) 209 is positioned at the position of the standard microphone 201.

Measure sound pressure (p _m ) with a sound level meter (SLM) 209 under the same conditions as in step 3 and step 1.

Then, p _m and p _t are compared to perform calibration.

The algorithm for calibration of the sound field is as follows.

Calibration of the sound source field involves the process of finding the speaker input voltage (V ^s ) at each frequency that produces a target sound pressure (p _t ) that is frequency invariant. Assuming that the speaker responds linearly to V ^s at a given frequency, the radiated conductance β can be written as β (f) = p _t / V ^s (f) when V ^s is a frequency dependent variable.

The voltage output V ₀ of the standard microphone 201 is given by V ₀ = Sp using the microphone's sensitivity S to sound pressure.

The voltage V ₂ measured by the DVM II 203 of FIGS. 4 and 5 is the microphone sensitivity S and the overall gain G _e of the network including the DVM II 203, the main amplifier 202, the preamplifier and the like. The sound pressure at a given frequency f can be written as

p (f) = V ₂ (f) / S (f) G _e (f) --------- (1)

The measured sound pressure level SPL _m corresponding to the sound pressure p _m = V ₂ / SG _e at the measuring position can be expressed as a function of V ₂ .

SPL _m = 20log (V ₂ / SG _e p _ref ) --------- (2)

Where p _ref is 20μPa.

Assuming that the entire calibration system, including the measuring and driving units, has good linearity, substituting β (f) = p _t / V ^s (f) into Eq. (1) and setting α = SG _e β, the measured voltage (V ₂ ) is expressed as follows.

V ₂ = αV ^s ---------- (3)

If the speaker output voltage V ^s is maintained in units for frequency changes, the voltage V ₂ is equal to α, so α (f) can be seen as the frequency response of the entire calibration system, and the sensitivity S and gain G _e are invariant with frequency. Is the frequency response of the speaker. Here, when formula (1) and formula (3) are combined, it can be seen that the sound pressure is proportional to the voltage supplied to the speaker.

That is, p∝V ^s to be.

Therefore, the speaker input voltage required to obtain the target sound pressure p _t using the above relationship can be obtained according to the following cyclical formula.

V ^s _{i +1} = V ^s _i p _t / p _i ---------(4)

Where i and i + 1 represent the i th and i + 1 th repeats, respectively. The loudspeaker input voltage determination method according to the invention by the circular equation (4) based on linear system characteristics shows very fast convergence characteristics.

If, during long-term or temporary operation, there is a displacement in the internal linear characteristics, the loudspeaker input voltage solution by the circular equation (4) becomes inefficient, and a complementary solution is necessary. Linear equations with a slight displacement can be assumed to be close to quadratic in a narrow region.

Three test speaker input voltages V ^s _{i -2} ,, V ^s _{i -1} , V ^s _i Considering that (i is 3, 4, 5, ...) was attempted, if f (V _s ) = p (V ^s ) -p _t is the difference between the target sound pressure and the target value, It is possible.

f (V ^s ) = a (V ^s -V ^s _i ) ² + b (V ^s -V ^s _i ) + c --------- (5)

Where a, b, c are the three speaker input voltages V ^s _{i -2} , V ^s _{i -1} , V ^s _i (i can be determined by substituting 3, 4, 5, ...) into equation (5).

In other words,

-------(6)

------- (6)

V ^s _{i + 1} is found by f (V ^s _{i + 1} ) = 0 and if you solve the quadratic equation

-----------(7)

----------- (7)

Equation (7) is a relational equation that can be used in addition to Equation (4) when there is a displacement to the linear characteristics of the system. However, equation (7) is only used when b ² ≥ 4ac, otherwise the equation (4) is used until this condition is met. In this process, the speaker input voltage of the computer controller is applied by using the circular equation (4) for the first three attempts in the search algorithm, and using the circular equation (7) from the next attempt.

The algorithm for calibration of the sound level meter is as follows.

The speaker input voltage (V ^s ) irradiated by the operation described above is applied to the sound level meter as an invariant constant for frequency in the acoustic field. Two output terminal voltages of the sound level meter V _DC and V _AC Is proportional to sound pressure level-indication and sound pressure. Therefore, the relationship between the sound pressure level SPL and the terminal voltage is as follows.

SPL = R _DC V _DC --- (8)

SPL = 20logV _AC + R _AC --- (9)

Where R _AC and R _DC are the attenuation coefficients for the AC and DC outputs of the sound level meter, respectively. Comparing Expression (2) with Expression (9), the coefficient R _AC is obtained as follows.

R _AC = 20log (p _ref SG _e ) ----------- (10)

The sensitivity S is determined by the interactivity method as a function of frequency, and the effective gain G _e of equation (10) can be measured by comparing the input voltage to the SLM and the output voltage of the AC terminal. Since the effective gain and sensitivity of a precision sound level meter do not depend on the frequency, it is assumed that the attenuation coefficient (R _AC ) measured at a single frequency (1 kHz) is generally valid over the entire frequency range.

Validation of the present invention

As a result of performing frequency correction and calibration by applying a single frequency attenuation coefficient according to the present invention, it can be seen that the frequency response of the precision sound level meter (silencer) is within the allowable range in the entire target frequency region as shown in FIG. . Therefore, the sound level meter calibration method according to the present invention using a single damping coefficient has proven to be reliable.

Indeed, in most cases where the linearity of the calibration system is good, the target sound pressure can be achieved on the second sound field calibration attempt.

In order to test the linearity of the system , the sound pressure level (SPL) and the measured amplifier AC voltage (V ₂ ) were investigated as a function of the picker input voltage (V ^s ) and the frequency (f). As shown in FIG. 7, all the intervals between the curves represented equal intervals. However, there was an irregular gap between 0.6V and 0.9V near the 3.15 ~ 4.0kHz region of the speaker's crossover frequency. This is the result of demonstrating that the linearity assumption is generally suitable for a wide range of frequencies, except that the calibration system of the present invention excludes the cross frequency range from the low speaker input voltage range.

Reconstructing the data shown in Fig. 7 as a function of the speaker input voltage is shown in Figs.

Fig. 8 (a) is a graph showing the output voltage V ₂ as a function of the speaker input voltage V ^s at or below the crossover frequency of the speaker (about 3.4 kHz), where good linearity is observed. Fig. 9A is a graph showing the output voltage V ₂ as a function of the speaker input voltage V ^s in the cross-frequency region, where nonlinearity is observed. Here, the line slope of the graph corresponds to the proportional constant α of Equation (3), and can be obtained by the least square method as a ratio of the digital voltmeter indication value to the speaker input voltage. Fig. 8 (b) shows the logarithmic proportionality of the sound pressure level with respect to the speaker input voltage. In the case of Fig. 9 (b), a deviation from the logarithmic proportionality is observed as the crossover frequency range.

The linearity observed in FIG. 8 demonstrates the validity of the assumption on which the calibration method algorithm according to the invention is based. The non-linearity results shown in FIG. 9 show that a supplemental procedure by cyclic equation (7) is required for the calibration procedure.

Calibration of the sound level meter is accomplished by comparing the sound pressure level (SPL) measured by the sound level to be calibrated with the sound pressure level of the standard microphone.

The present invention provides an automatic calibration system and calibration method that allows for quick calibration within at least ± 0.01 dB in an anechoic chamber.

Applying the algorithm of the system according to the present invention, the repetitive number of times of reaching the target sound pressure is very small due to the fast convergence characteristic. In the calibration according to the present invention, fast convergence can be obtained with an accuracy of a small error range of ± 0.01 dB corresponding to 0.01% for 85 dB reference or ± 4.1 μPa corresponding to 0.001% for 0.356Pa reference, A stable result was shown.

Claims (9)

A driver comprising a signal generator for generating a sine signal applied to a standard sound source and a driver digital voltmeter for counting the frequency of the sine signal applied to the standard sound source and measuring voltage; and amplifying the output signal of the standard microphone or sound level meter. A method for calibrating a sound level meter using a sound level meter calibration system comprising: a measuring part including a measuring amplifier and a measuring part digital voltmeter measuring the output voltage of the amplifier. (a) installing the standard microphone in a fixed position; (b) selecting an input voltage V ^s of a standard sound source by a signal generator; (c) applying a selective input voltage (V ^s ) to a standard sound source to apply a sound field to the standard microphone; (d) measuring the voltage (V ₂ ) generated by the standard microphone by the measuring unit digital voltmeter and recording the voltage value; (e) calculating and recording a sound pressure _pi corresponding to the voltage value recorded in step (d); (f) the target pressure if the tolerance within the range of the sound pressure (p _i) is the target pressure (p _t) recorded at the step (e) (p _t) input voltage (V ^SRC standard sound source are determined and corresponding to the obtained Recording); (g) If the target sound pressure is not obtained in step (f), the new selected input voltage V ^s _{i + 1} of the standard sound source is cyclic V ^s _{i +1} = V ^s _i (p _t / p _i ). Calculating i (where i denotes the current repeat count and i + 1 indicates the next repeat count), and returning to step (c); Sound meter calibration method, characterized in that for adjusting the input voltage applied to the standard sound source. The method of claim 1, If the repetition number (i) in step (g) is 3 or more, replace the cyclic formula for calculating the selection input voltage (V ^s _{i +1} ) of the standard sound source by the following formula (c) to step (g) A sound level meter calibration method comprising:

here

,

,

,

. The sound level meter calibration method according to claim 1 or 2, wherein the steps (b) to (g) are repeatedly performed by changing a frequency in a target frequency range. The sound level meter calibration method according to claim 1 or 2, wherein the steps (b) to (g) are automatically executed under the control of a computer. 4. The sound level meter calibration method according to claim 3, wherein the repetitive performance of changing the frequency is automatically performed under the control of a computer. The method according to claim 1 or 2 (h) removing the standard microphone and installing the calibration sound level meter in the same position, (i) applying a sound field to the sound level meter by applying the input voltage V ^SRC of the standard sound source determined in step (f) to the standard sound source; (j) measuring the voltage generated by the sound level meter (V ₂ ) by the measuring unit digital voltmeter and recording the voltage value; (k) calculating and recording the measured sound pressure p _m corresponding to the voltage value recorded in step (j); (l) comparing the target sound pressure p _t with the measured sound pressure p _m to perform calibration of the sound level meter; Sound level meter calibration method further comprising a. delete delete delete

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