An active coupled cavity device and method for calibrating a working standard microphone

Abstract

This invention discloses an active coupling cavity device and calibration method for calibrating a standard microphone. A sound source adapter is located at the signal input end of the active coupling cavity. A multi-channel sound field distribution structure is located within the active coupling cavity, comprising eight acoustic channels evenly distributed circumferentially along the central axis of the active coupling cavity. A symmetrical clamping assembly constrains the reference microphone and the microphone under test to be symmetrical about the central axis of the active coupling cavity. A coaxiality test ring assembly rigidly constrains the axes of the reference microphone and the microphone under test to coincide with the central axis of the active coupling cavity. The calibration test system includes a signal generator, a sound level meter under test, a preamplifier, a measurement amplifier, a bandpass filter, and a digital voltmeter. The multi-channel uniformly distributed sound field structure solves the problem of uneven sound pressure distribution caused by unidirectional input, and the symmetrical clamping assembly and the coaxiality test ring assembly respectively solve the problem of poor consistency in low-frequency and high-frequency phase measurements.

Description

Technical Field

[0001] This invention relates to the field of acoustic metrology and calibration technology, specifically to an active coupling cavity device and calibration method for calibrating a working standard microphone. Background Technology

[0002] With the development of science and technology and industrial production, increasingly stringent environmental protection and noise control requirements have led to a significant increase in the demand for working standard microphones. Today, working standard microphones are widely used in key industries that require precise acoustic testing, such as outdoor noise monitoring, sound source localization, automobiles, and large aircraft. To ensure the accuracy of test data, the traceability of microphone values ​​is particularly crucial. According to the requirements of the international standard IEC61094-2:2009 and the national metrological verification procedure JJG1019-2007 "Working Standard Microphones (Coupled Cavity Comparison Method)," the precise calibration of microphones requires the use of an active coupling cavity, and this coupling cavity must have the ability to generate a radially symmetrical sound field within the calibrated frequency range, with a sound pressure level within the cavity not less than 80 dB.

[0003] Currently, the existing coupled-unit method has significant drawbacks when measuring the frequency response and phase of a microphone: First, different microphones have different equalization methods. When using the current method, the equalization hole of the microphone, such as the post-equalization type, may not be fully exposed in the sound field of the coupling unit, resulting in distorted measurement results. Secondly, traditional equipment typically uses a single channel to guide the sound signal into the coupling unit at the input end. Since the microphone diaphragm receives the signal in a circular manner, the distribution of the unidirectional sinusoidal sound wave in different areas of the microphone diaphragm is extremely uneven. This directly leads to the system being unable to accurately reflect the microphone's true frequency response and phase characteristics.

[0004] When performing microphone comparison calibration, the uniformity of the sound field and the phase consistency inside the coupling cavity are easily disrupted by physical structural asymmetry. In actual calibration operations at low and high frequencies, different physical constraint problems are faced. On the one hand, during the random insertion of microphones, the existing clamping components cannot guarantee that the standard microphone and the diaphragm of the microphone under test maintain a very small and strictly symmetrical distance, which directly leads to poor consistency of repeated measurements. On the other hand, machining tolerances and clamping deviations can cause asymmetry in the cavity structure. During high-frequency testing, minute structural asymmetries (such as tilted or protruding defects on the coupling surface) can not only cause phase interference effects of sound waves, but also produce geometric projection effects, resulting in severe distortion of the high-frequency sound field and significant errors in the frequency response of the coupling surfaces on both sides.

[0005] In summary, traditional single-channel input and conventional clamping designs cannot simultaneously achieve absolute uniformity of the sound field and meet the high-precision testing requirements across the entire frequency range. There is an urgent need to develop a new type of active coupling cavity device that can achieve a multi-channel uniformly distributed sound field and has a high-precision symmetrical clamping and coaxiality constraint mechanism to solve the calibration error problems caused by sound field non-uniformity and assembly tolerance. Summary of the Invention

[0006] This invention proposes an active coupling cavity device and calibration method for calibrating a standard microphone. It solves the problem of uneven sound pressure distribution caused by unidirectional input by using a multi-channel uniformly distributed sound field structure, and solves the problem of poor consistency in low-frequency and high-frequency phase measurements by using a symmetrical clamping component and a coaxiality test ring component, respectively, thereby achieving accurate and stable measurement of microphone sound pressure and phase.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows: An active coupling cavity device for calibrating a standard microphone includes an active coupling cavity, a sound source adapter, a multi-channel sound field distribution structure, a symmetrical clamping assembly, a coaxiality test ring assembly, and a calibration test system. The sound source synergist is located at the signal input end of the active coupling cavity and is used to receive external excitation sound signals; The multi-channel sound field distribution structure is located in the inner cavity of the active coupling cavity. The multi-channel sound field distribution structure includes eight acoustic channels that are evenly distributed circumferentially along the central axis of the active coupling cavity. The input ends of the eight acoustic channels converge at the output end of the sound source combiner. The output ends of the eight acoustic channels are evenly distributed 360° along the inner cavity of the active coupling cavity. This structure is used to divide the unidirectional sound signal input from the sound source combiner into eight synchronous sound signals, forming a radially symmetrical sound field inside the active coupling cavity. The symmetrical clamping assembly includes a first clamping part and a second clamping part, which are located at the two ends of the active coupling cavity, respectively. The first clamping part is used to limit the reference microphone, and the second clamping part is used to limit the microphone under test. The symmetrical clamping assembly constrains the reference microphone and the microphone under test to be symmetrical about the central axis of the active coupling cavity. The coaxiality test ring assembly includes a first test ring and a second test ring. The first test ring is threadedly connected to the front end of the reference microphone, and the second test ring is threadedly connected to the front end of the microphone under test. Both the first test ring and the second test ring are coaxially clearance-fitted with the inner hole of the active coupling cavity to rigidly constrain the axes of the reference microphone and the microphone under test to coincide with the central axis of the active coupling cavity. The calibration test system includes a signal generator, a sound level meter under test, a preamplifier, a measurement amplifier, a bandpass filter, and a digital voltmeter. The output of the signal generator is connected to the excitation sound source of the sound source synchro. The output of the active coupling cavity is connected to the signal inputs of the reference microphone and the microphone under test, respectively. The output of the microphone under test is connected to the input of the sound level meter under test. The output of the reference microphone is connected to the input of the preamplifier. The output of the preamplifier is connected to the digital voltmeter in sequence through the measurement amplifier, the bandpass filter unit, and the measurement amplifier.

[0008] Furthermore, the first clamping part and the second clamping part are respectively connected to the end flange of the active coupling cavity. The center of the first clamping part and the second clamping part is provided with a positioning hole adapted to the reference microphone and the preamplifier of the microphone under test. The axis of the positioning hole coincides with the central axis of the active coupling cavity.

[0009] Furthermore, the positioning holes of the first clamping part and the second clamping part are provided with stepped limiting surfaces, which are in contact with the mounting end faces of the reference microphone and the microphone under test, and are used to control the diaphragm distance between the reference microphone and the microphone under test.

[0010] Furthermore, the active coupling cavity, the symmetrical clamping assembly, and the coaxiality test ring assembly are detachably connected by a positioning stop and flange bolts.

[0011] An active coupler calibration method for calibrating a standard microphone, based on the aforementioned active coupler device, includes the following steps: Step S1) System warm-up and initialization: Turn on the electronic instruments of the calibration test system, such as the signal generator, preamplifier and the sound level meter being calibrated, to warm up, and confirm that the temperature and humidity of the test environment meet the calibration specifications. Step S2) Install the first test ring and the second test ring at the front end of the reference microphone and the microphone under test, respectively. Install the reference microphone and the microphone under test at the first clamping part and the second clamping part at both ends of the active coupling cavity, respectively. Constrain the diaphragm spacing of the reference microphone and the microphone under test to be less than 0.4 mm by the symmetrical clamping assembly. Constrain the reference microphone and the microphone under test to be coaxial with the active coupling cavity by the coaxiality test ring assembly. Step S3) The excitation sound signal is input into the active coupling cavity through the sound source combiner. The excitation sound signal is divided into eight synchronous sound signals through the eight acoustic channels of the multi-channel sound field distribution structure, forming a 360° radially symmetrical sound field inside the coupling cavity, so that the diaphragms and equalizing holes of the reference microphone and the microphone under test are fully exposed in the uniform sound field. Step S4) Control the excitation sound signal to sweep the frequency within the calibration frequency band of 20Hz~20000Hz, and simultaneously acquire the sound signals output by the reference microphone and the microphone under test. Step S5) The sound signal of the microphone under test is output to the sound level meter under test, and the sound level and electrical signal values ​​of the microphone under test are output and displayed by the sound level meter under test. The acoustic signal from the reference microphone is pre-amplified, measured, and band-pass filtered. Then, it is converted from analog to digital and stored by a digital voltmeter to obtain the standard sound pressure value of the active coupled cavity sound field. Step S6) Calculate the performance deviation of the microphone under test in each frequency band based on the output values ​​of the sound level meter and digital voltmeter. Step S7) After the test is completed, separate the reference microphone, the microphone under test and the coupling cavity device, and reset the device.

[0012] Compared with the prior art, the present invention has the following advantages: The 1-to-8 multi-channel uniform sound field structure completely solves the problem of uneven sound pressure distribution caused by traditional single-channel input. Through eight circumferentially uniformly distributed acoustic channels, a single input sound signal is divided into eight synchronous signals, so that the sinusoidal sound signal is uniformly covered on the microphone diaphragm from 360°. This not only realizes the construction of a radially symmetrical sound field in the coupling cavity, but also ensures that the microphone equalization holes with different equalization methods are fully exposed in the uniform sound field, thus avoiding the problem of measurement distortion at the source.

[0013] A graded mechanical limiting scheme is proposed, relying on spacing for low frequencies and coaxiality for high frequencies, to achieve high-precision measurement across the entire frequency band from 20Hz to 20000Hz. Addressing the characteristics of long wavelengths and small signal reception ratios of low-frequency sound waves, a symmetrical clamping assembly is used to strictly constrain the spacing between the microphone diaphragms on both sides to within 0.4mm, providing a symmetrical and ideal sound field environment for low-frequency calibration and eliminating low-frequency phase measurement deviations. For the characteristics of short wavelengths and extreme sensitivity to geometric asymmetry in high-frequency sound waves, a coaxiality test ring is used to rigidly constrain the microphones on both sides to the same axis, avoiding high-frequency sound field distortion and phase interference caused by installation tilt, and improving the repeatability and accuracy of high-frequency phase measurements.

[0014] The modular design, which integrates coupling, clamping, and testing, significantly reduces the measurement uncertainty of the whole machine. The split modular structure allows for flexible replacement of adapter components for different microphone models, avoiding the accumulation of processing errors in the integrated structure and reducing the interference of geometric deviations on the sound field.

[0015] This invention combines acoustic-structure coupling theory with the finite element method to perform full-parameter simulation optimization of the coupling cavity structure. By constructing acoustic-structure coupling dynamic equations, it quantitatively analyzes the interference law of the asymmetric structure on the sound field, ensuring the accuracy and stability of the coupling cavity device in actual measurement and meeting the relevant standard requirements for microphone calibration. Attached Figure Description

[0016] Figure 1 This is a cross-sectional structural schematic diagram of the active coupling cavity device of the present invention; Figure 2 This is a system architecture diagram of the calibration and testing system of the present invention; Figure 3 This is a flowchart of the finite element acoustic field analysis steps for the active coupling cavity of the present invention; Figure Labels 1. Active coupling cavity, 2. Acoustic channel, 3. Sound source connector, 4 First clamping part, 5 Second clamping part, 6 First test ring, 7 Second test ring. Detailed Implementation

[0017] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] This embodiment proposes an active coupling cavity device for calibrating a standard microphone, such as... Figure 1 As shown, it includes an active coupling cavity 1, a sound source mate 3, a multi-channel sound field distribution structure, a symmetrical clamping assembly, a coaxiality test ring assembly, and a calibration test system.

[0019] The sound source synergist 3 is installed on the signal input end of the side wall of the active coupling cavity 1. It has an excitation sound source inside. The input end of the sound source synergist 3 is connected to an external signal generator, and the output end is connected to a multi-channel sound field distribution structure.

[0020] The multi-channel sound field distribution structure is located inside the active coupling cavity 1. The multi-channel sound field distribution structure includes eight acoustic channels 2 that are evenly distributed circumferentially along the central axis of the active coupling cavity 1. The input ends of the eight acoustic channels 2 converge at the output end of the sound source combiner 3. The output ends of the eight acoustic channels 2 are evenly distributed 360° along the inner cavity of the active coupling cavity 1. This structure is used to divide the unidirectional sound signal input from the sound source combiner 3 into eight synchronous sound signals, ensuring that the eight sound signals are output synchronously and with equal amplitude, thus forming a radially symmetrical uniform sound field inside the active coupling cavity 1.

[0021] The symmetrical clamping assembly includes a first clamping part 4 and a second clamping part 5. The first clamping part 4 and the second clamping part 5 are located at both ends of the active coupling cavity 1, respectively. A positioning stop is provided between the first clamping part 4, the second clamping part 5 and the end of the active coupling cavity 1 to ensure coaxiality during installation.

[0022] The first clamping part 4 is used to limit the reference microphone, and the second clamping part 5 is used to limit the microphone under test. The symmetrical clamping assembly constrains the reference microphone and the microphone under test to be symmetrical about the central axis of the active coupling cavity 1.

[0023] The first clamping part 4 and the second clamping part 5 are respectively connected to the end flange of the active coupling cavity 1. The center of the first clamping part 4 and the second clamping part 5 is provided with a positioning hole adapted to the microphone preamplifier stage. The axis of the positioning hole coincides with the central axis of the active coupling cavity 1.

[0024] The positioning holes of the first clamping part 4 and the second clamping part 5 are provided with stepped limiting surfaces, which fit in contact with the mounting end face of the microphone to control the diaphragm distance between the reference microphone and the microphone under test.

[0025] The coaxiality test ring assembly includes a first test ring 6 and a second test ring 7. The inner hole of the first test ring 6 is provided with a mounting thread, and the first test ring 6 is screwed into the thread of a reference microphone. The inner hole of the second test ring 7 is provided with a mounting thread, and the second test ring 7 is screwed into the thread of the microphone under test. Both the first test ring 6 and the second test ring 7 are coaxially clearance-fitted with the inner hole of the active coupling cavity 1, and the axes of the rigidly constrained reference microphone and the microphone under test are completely coincident with the central axis of the active coupling cavity 1, eliminating the problem of high-frequency sound field distortion caused by the tilt of the reference microphone installation.

[0026] like Figure 2 As shown, the calibration test system includes a signal generator, a sound level meter to be calibrated, a preamplifier, a measuring amplifier, a bandpass filter, and a digital voltmeter. The output of the signal generator is connected to the excitation sound source of the sound source matching unit 3. The signal generator can be a DG4102 type signal generator, whose output is connected to the excitation sound source of the sound source matching unit 3, and is used to output a sinusoidal excitation signal of 20Hz~20000Hz.

[0027] The output of the active coupling cavity 1 is connected to the signal input of the reference microphone and the microphone under test, respectively. The output of the microphone under test is connected to the input of the sound level meter being calibrated. The output of the reference microphone is connected to the input of the preamplifier. The output of the preamplifier is connected to the digital voltmeter in sequence through the measuring amplifier, the bandpass filter unit, and the measuring amplifier.

[0028] In actual installation, the active coupling cavity 1, the symmetrical clamping assembly and the coaxiality test ring assembly are detachably connected by positioning stops and flange bolts.

[0029] A calibration method for an active coupling cavity 1 used for calibrating a standard microphone, based on the aforementioned active coupling cavity device, includes the following steps: Step S1) System warm-up and initialization: Turn on the electronic instruments of the calibration test system, such as the signal generator, preamplifier and the sound level meter being calibrated, to warm up, and confirm that the temperature and humidity of the test environment meet the calibration specifications. Step S2) Install the first test ring 6 and the second test ring 7 on the standard threads at the front end of the reference microphone and the microphone under test respectively by threaded connection. Install the reference microphone and the microphone under test in the positioning holes of the first clamping part 4 and the second clamping part 5 at both ends of the active coupling cavity 1 respectively, so that the mounting end face of the reference microphone and the microphone under test is completely in contact with the step limiting surface in the positioning hole. The diaphragm spacing of the reference microphone and the microphone under test is constrained to be less than 0.4mm by the symmetrical clamping assembly. The coaxiality test ring assembly constrains the reference microphone and the microphone under test to be coaxial with the active coupling cavity 1. Step S3) The excitation sound signal is input into the active coupling cavity 1 through the sound source synergist 3. The excitation sound signal is divided into eight synchronous sound signals through the eight acoustic channels 2 of the multi-channel sound field distribution structure, forming a 360° radially symmetrical sound field inside the coupling cavity, so that the diaphragm and equalizing hole of the reference microphone and the microphone under test are fully exposed in the uniform sound field, thus constructing an ideal calibration environment. Step S4) Control the excitation sound signal to sweep the frequency within the calibration frequency band of 20Hz~20000Hz, and simultaneously acquire the sound signals output by the reference microphone and the microphone under test. Step S5) The sound signal of the microphone under test is output to the sound level meter under test. The sound level and electrical signal values ​​measured by the microphone under test are output and displayed by the sound level meter under test, and the values ​​are transmitted to the host computer in real time. The acoustic signal from the reference microphone is preamplified, measured, and bandpass filtered. It is then converted from analog to digital and stored by a digital voltmeter. The value is transmitted to the host computer in real time to obtain the standard sound pressure value of the sound field of the active coupling cavity 1. Step S6) Based on the values ​​output by the sound level meter and digital voltmeter, the host computer calculates the performance deviation of the microphone under test in each frequency band. The host computer processes the collected full-frequency data, uses the value of the reference microphone as a reference, calculates the frequency response deviation and phase deviation of the microphone under test, and generates a calibration report that conforms to the metrological specifications. Step S7) Control signal generator to stop outputting excitation sound signal, test completed, separate reference microphone, microphone under test from active coupling cavity 1 device, and reset device.

[0030] In this embodiment, during the design phase, the device was fully parametrically simulated and optimized using the finite element method, based on acoustic-structure interaction theory. The specific steps are as follows: Figure 3 As shown: Preprocessing stage: Clarify the specific problems to be analyzed, namely the interference law of the sound field uniformity and structural asymmetry of the active coupling cavity 1 on the sound field; Preprocessing stage: Establish a three-dimensional parametric geometric model including active coupling cavity 1, mechanical structure and microphone coupling interface, set material parameters and finite element parameters, divide the mesh, and construct the acoustic-structure coupling dynamic equation; Solution phase: Set a wideband excitation signal and perform simulation and parameter optimization on the acoustic field aperture and acoustic field channel inside the active coupling cavity 1; Post-processing stage: The solution results are analyzed, and the interference law of asymmetric factors such as inclined surfaces and local protrusions on the mid-to-high frequency sound field response is quantitatively analyzed. The physical mechanism of sound field distortion caused by machining deviation is revealed. Finally, scientific boundaries are set for the machining and assembly tolerance of clamping mechanism and test components to ensure the high uniformity of sound pressure distribution in the cavity and the phase consistency of the calibration surfaces on both sides under broadband excitation.

[0031] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An active coupling cavity device for calibrating a standard microphone, characterized in that, It includes an active coupling cavity, a sound source mate, a multi-channel sound field distribution structure, a symmetrical clamping assembly, a coaxiality test ring assembly, and a calibration test system; The sound source synergist is located at the signal input end of the active coupling cavity and is used to receive external excitation sound signals; The multi-channel sound field distribution structure is located in the inner cavity of the active coupling cavity. The multi-channel sound field distribution structure includes eight acoustic channels that are evenly distributed circumferentially along the central axis of the active coupling cavity. The input ends of the eight acoustic channels converge at the output end of the sound source combiner. The output ends of the eight acoustic channels are evenly distributed 360° along the inner cavity of the active coupling cavity. This structure is used to divide the unidirectional sound signal input from the sound source combiner into eight synchronous sound signals, forming a radially symmetrical sound field inside the active coupling cavity. The symmetrical clamping assembly includes a first clamping part and a second clamping part, which are located at the two ends of the active coupling cavity, respectively. The first clamping part is used to limit the reference microphone, and the second clamping part is used to limit the microphone under test. The symmetrical clamping assembly constrains the reference microphone and the microphone under test to be symmetrical about the central axis of the active coupling cavity. The coaxiality test ring assembly includes a first test ring and a second test ring. The first test ring is threadedly connected to the front end of the reference microphone, and the second test ring is threadedly connected to the front end of the microphone under test. Both the first test ring and the second test ring are coaxially clearance-fitted with the inner hole of the active coupling cavity to rigidly constrain the axes of the reference microphone and the microphone under test to coincide with the central axis of the active coupling cavity. The calibration test system includes a signal generator, a sound level meter under test, a preamplifier, a measurement amplifier, a bandpass filter, and a digital voltmeter. The output of the signal generator is connected to the excitation sound source of the sound source synchro. The output of the active coupling cavity is connected to the signal inputs of the reference microphone and the microphone under test, respectively. The output of the microphone under test is connected to the input of the sound level meter under test. The output of the reference microphone is connected to the input of the preamplifier. The output of the preamplifier is connected to the digital voltmeter in sequence through the measurement amplifier, the bandpass filter unit, and the measurement amplifier.

2. The active coupling cavity device for calibrating a standard microphone according to claim 1, characterized in that, The first clamping part and the second clamping part are respectively connected to the end flange of the active coupling cavity. The center of the first clamping part and the second clamping part is provided with a positioning hole adapted to the reference microphone and the preamplifier of the microphone under test. The axis of the positioning hole coincides with the central axis of the active coupling cavity.

3. The active coupling cavity device for calibrating a standard microphone according to claim 2, characterized in that, The positioning holes of the first clamping part and the second clamping part are provided with stepped limiting surfaces. The limiting surfaces are in contact with the mounting end faces of the reference microphone and the microphone under test, and are used to control the diaphragm distance between the reference microphone and the microphone under test.

4. The active coupling cavity device for calibrating a standard microphone according to claim 1, characterized in that, The active coupling cavity, the symmetrical clamping assembly, and the coaxiality test ring assembly are detachably connected by a positioning stop and flange bolts.

5. An active coupling cavity calibration method for calibrating a standard microphone, implemented based on the active coupling cavity device according to any one of claims 1-4, characterized in that, Includes the following steps: Step S1) System warm-up and initialization: Turn on the electronic instruments of the calibration test system, such as the signal generator, preamplifier and the sound level meter being calibrated, to warm up, and confirm that the temperature and humidity of the test environment meet the calibration specifications. Step S2) Install the first test ring and the second test ring at the front end of the reference microphone and the microphone under test, respectively. Install the reference microphone and the microphone under test at the first clamping part and the second clamping part at both ends of the active coupling cavity, respectively. Constrain the diaphragm spacing of the reference microphone and the microphone under test to be less than 0.4 mm by the symmetrical clamping assembly. Constrain the reference microphone and the microphone under test to be coaxial with the active coupling cavity by the coaxiality test ring assembly. Step S3) The excitation sound signal is input into the active coupling cavity through the sound source combiner. The excitation sound signal is divided into eight synchronous sound signals through the eight acoustic channels of the multi-channel sound field distribution structure, forming a 360° radially symmetrical sound field inside the coupling cavity, so that the diaphragms and equalizing holes of the reference microphone and the microphone under test are fully exposed in the uniform sound field. Step S4) Control the excitation sound signal to sweep the frequency within the calibration frequency band of 20Hz~20000Hz, and simultaneously acquire the sound signals output by the reference microphone and the microphone under test. Step S5) The sound signal of the microphone under test is output to the sound level meter under test, and the sound level and electrical signal values ​​of the microphone under test are output and displayed by the sound level meter under test. The acoustic signal from the reference microphone is pre-amplified, measured, and band-pass filtered. Then, it is converted from analog to digital and stored by a digital voltmeter to obtain the standard sound pressure value of the active coupled cavity sound field. Step S6) Calculate the performance deviation of the microphone under test in each frequency band based on the output values ​​of the sound level meter and digital voltmeter. Step S7) After the test is completed, separate the reference microphone, the microphone under test and the coupling cavity device, and reset the device.

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