Laboratory Standard Microphones Research Articles

Toward a unified formulation of the acoustic transfer admittance of cylindrical cavities for reciprocity calibration of microphones

The pressure reciprocity technique for calibration of Laboratory Standard microphones provides the basis for primary measurement standards for sound pressure. This calibration method is described in the International Electrotechnical Commission (IEC) standard 61094-2:2009 where the key aspect relating to the calculation of the acoustic transfer admittance of couplers was recently completed and clarified in the amendment IEC 61094-2:2009/AMD1:2022. Three models are currently provided for calculating this quantity: the low-frequency solution suitable to low frequencies, the extended low-frequency solution suitable to low and medium frequencies, and the broadband solution suitable to medium and high frequencies. It is established that neither of these models is correct for all frequencies routinely considered in many National Metrology Institutes, namely from 2 Hz to 25 kHz, so that a transition must be made. Based on the fundamental equations of acoustics in thermoviscous fluid, this paper provides a unified formulation for the acoustic transfer admittance of cylindrical cavities. Under the common quasi-plane wave approximation, this paper provides a new form of solution for the Fourier equation, not restricted by any assumption on the pressure variation and leading to a new form of propagation equation in the coupler. Two techniques are used to solve the problem, the Laplace Adomian Decomposition Method and a stepped duct approximation technique. Numerical results are presented, providing substantial evidence to support the validity of these formulations. A high level of agreement is observed between the two models, approximately 10−6 dB for the amplitude and 10−4 degrees for the phase, excluding the resonance frequencies of couplers. Finally, this paper evaluates the compatibility of the extended low-frequency solution provided in the IEC amendment with the fundamental equations established in this study. A new simplified solution is provided, highlighting discrepancies in the application of the heat conduction corrective factor as compared to the guidelines outlined in the IEC amendment.

Final report on the key comparison CCAUV.A-K6

Main textThe key comparison CCAUV.A-K6 has been carried out under the auspices of the Consultative Committee on Acoustics, Ultrasound and Vibration (CCAUV) of the International Committee of Weights and Measures (CIPM). This comparison is concerned with primary pressure calibration of laboratory standard microphones type LS2P. The participating NMI's are HBK-DPLA (Denmark), CENAM (Mexico), GUM (Poland), INMETRO (Brazil), KRISS (Korea), LNE (France), METAS (Switzerland), NMIA (Australia), NMIJ (Japan), NMISA (South Africa), NRC (Canada), UME (Turkey) and VNIIFTRI (Russia). The role of Pilot laboratory was undertaken by LNE (France). The measurements took place between March 2019 and December 2020. Two LS2P microphones were circulated. This report includes the measurement results from the participants, information about their calibration methods, and the analysis leading to the assignment of the Key Comparison Reference Values (KCRV) and Degrees of Equivalence (DoE).To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCAUV, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

A laser pistonphone designed for absolute calibration of infrasound sensors from 10 mHz up to 20 Hz

There has been an increased demand for traceable calibrations at infrasonic frequencies in support of geophysical monitoring applications, an example being the Comprehensive Nuclear-Test-Ban Treaty Organization, which provides global international coverage for a nuclear testing ban, and requires the International Monitoring System. In this paper, a new laser pistonphone design is presented with the objective of establishing primary standards for sound pressure at very low frequencies down to 10 mHz. The piston is a modified accessorized loudspeaker driver whose diameter is equal to the diameter of the front pistonphone cavity. The volume velocity of the piston is measured through a laser interferometer and the current version was designed to have an upper frequency limit of 20 Hz, to overlap with the closed coupler reciprocity method of calibration. Particular attention has been given to the sealing to avoid pressure leakage loss. The dimensions of the front cavity were designed to allow the calibration of a large variety of sensors, including microphones, barometers, manometers and microbarometers. Examples of calibrations for several sensors are presented and also an uncertainty budget for the Brüel & Kjær type 4160 laboratory standard microphones, commonly used for primary calibrations. Finally, the metrological performance of the laser pistonphone is demonstrated by comparing the calibration results with those obtained with alternative methods.

Laser-based comparison calibration of laboratory standard microphones.

A precision laser-based comparison calibration method for laboratory standard microphones is described that uses reference microphones calibrated by the pressure reciprocity method. Electrical drive current and diaphragm velocity are measured while the microphones are driven as transmitters/sources of sound; the diaphragm velocity is measured using scanning laser Doppler vibrometry. Sensitivities determined using this method display very good agreement with those determined directly by reciprocity for seven such test microphones at 250 and 1000 Hz. At these frequencies, the expanded (coverage factor k = 2) uncertainties of this comparison calibration method for these microphones are ±0.05 dB.

Low-noise optical measurement of sound using midfringe locked interferometer with differential detection.

A midfringe locked interferometer with differential detection is proposed for non-contact optical sound measurement, and the equivalent noise level of approximately 0 dB SPL/Hz is achieved. The noise level of the proposed method is 30 dB lower than that of a very recent laser Doppler vibrometer and close to that of a quarter-inch measurement microphone. The midfringe locking stabilizes the optical interferometer against slow environmental fluctuations and enables detection of the acoustic signal directly from optical intensity. The differential detection method eliminates laser intensity noise, which is a dominant noise source in optical interferometers. The noise level of the constructed system was approximately 10 dB above the optical shot-noise (the classical detection limit attributed to the quantum nature of light) at frequencies higher than 2 kHz. Further noise reduction by several available methods could lead to optical measurements that are more sensitive than measurements by microphones. In addition, the constructed interferometer is used to reconstruct sound fields generated by a half-inch laboratory standard microphone used as a transmitter. The proposed method will be a powerful tool for measuring small-amplitude sound fields where it has been challenging to use existing methods.

Laser-based comparison calibration of laboratory standard microphones

A precision laser-based comparison calibration method for laboratory standard microphones is described that uses reference microphones calibrated by the pressure reciprocity method. Electrical drive current and diaphragm velocity are measured while the microphones are driven as transmitters/sources of sound; the diaphragm velocity is measured using scanning laser Doppler vibrometry. Sensitivities determined using this method display very good agreement with those determined directly by reciprocity for seven such test microphones at 250 and 1000 Hz. At these frequencies, the expanded (coverage factor k = 2) uncertainties of this comparison calibration method for these microphones are ±0.05 dB.

MICROPHONE FRONT CAVITY DEPTH MEASUREMENT USING NON-CONTACT METHOD AT NATIONAL MEASUREMENT STANDARDS – NATIONAL STANDARDIZATION AGENCY OF INDONESIA

<p>Front cavity depth of microphone has influence in the determination of laboratory standard microphone sensitivity. Therefore this parameter is included in the measurement uncertainty budget. To determine microphone sensitivity accurately, it is necessary to measure the actual front cavity depth instead of using nominal value. This paper explains the measurement of LS1P and LS2P standard microphone front cavity depth using optical depth measurement facilities at SNSU-BSN and its effect on determining microphone sensitivity. The measurement was obtained from 4 positions distributed over the diaphragm for each microphone. The front cavity depth measurement result for LS1P is 1,94 mm ± 0,01 mm and for LS2P is 0,48 mm ± 0,01 mm These results comply with IEC 61094 Measurement Microphones-Part 2: Primary Method for Pressure Calibration of Laboratory Standard Microphones by Reciprocity Technique as the results are within the permissible range.</p>

Final report: key comparison EURAMET.AUV.A-K5

This is the final (Draft B) report for key comparison EURAMET.AUV.A-K5 on the pressure calibration of laboratory standard microphones in the frequency range from 2 Hz to 10 kHz. Twelve national metrology institutes took part in the key comparison and the National Physical Laboratory piloted the project. Two travelling standard microphones were circulated to the participants and results in the form of regular calibration certificates were collected throughout the project. The analysis used the results for one of the microphones only and values for both sensitivity level and sensitivity phase and have been linked to the CCAUV.A K5 key comparison reference value (KCRV) via two linking laboratories (NPL and INRIM).KEY WORDS FOR SEARCHacoustics, microphones, metrology, key comparison, pressure calibrationMain textTo reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCAUV, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Free-field reciprocity calibration of measurement microphones at frequencies up to 150 kHz.

Microphones are typically calibrated in a free field at frequencies up to 40 kHz using primary and secondary methods. This upper frequency is sufficiently high as to cover most sound measurement applications related with airborne noise assessment. However, other applications such as measurement of noise emitted by ultrasound cleaning machines, failure detection in aeronautic structures, and the investigation of the perception mechanisms of ultrasound may require that the sensitivity of the microphone is known at frequencies up to 150 kHz. In any of these applications, it is critical to establish a well-defined traceability chain to SI units to support any measurement result. In order to extend the frequency range of absolute free-field calibration, typical reciprocity measurement systems and measurement methods must undergo a series of changes and adaptations which may include using other types of microphones rather than laboratory standard microphones, changing the type of measurement signal, improving the methods for eliminating unwanted reflections from walls, cross-talk, distortion, etc. Herein, a strategy for the changes and adaptations to the existing measurement methodologies, and the determination of the microphone parameters is outlined, the results of its implementation are discussed, and calibrations results are presented and discussed.

Sensitivity measurement of a laboratory standard microphone by measuring the diaphragm vibration

As a method to determine the sensitivity of a laboratory standard microphone, the reciprocity method is employed as an international standard (IEC61094-2). This method is highly reliable and achieves high precision. However, three microphones are essentially required for this method. This procedure is complicated and takes a long time due to many combinations of microphone pairs and couplers. To overcome these difficulties, a novel method based on the measurement of vibration on the surface of a microphone using a laser Doppler vibrometer is proposed. It aims to obtain the sensitivity of a laboratory standard microphone. To this end, the relationship between the surface velocity of the diaphragm centre and acoustic volume velocity induced by the vibration of the diaphragm surface is investigated, and an empirical model is developed by measuring the distribution of the surface velocity. This paper used an empirical model of a 1-in. laboratory standard microphone estimated with 6 samples. The estimated sensitivity using this model has a difference of about 0.1 dB from the results of the reciprocity method applied to samples not used for estimating the empirical model.

Validated data mining technique: a case study on pressure-field digital MEMS microphone calibration

Recently, in the field of environmental noise control and monitoring, a diffusion of a new generation of microphone sensors has been observed. This was met by the rapid development of MEMS (MicroElectroMechanical Systems) microphones, whose technology could open up the possibility of noise measurements based on extensive acoustic sensor networks, that use a large number of microphones spread across wide areas. Nevertheless, current standards do not provide proper calibration and test procedures for these microphones. In this work, calibration standard procedures was adapted for the complete characterization of digital MEMS microphones by comparison with laboratory standard microphones in pressure field condition. Large data-set obtained from comparison, is subjected to a statistical analysis in order to highlight any possible factor affecting calibration accuracy and robustness.

Emerging alternative to the well-known LS2P microphones

The usual primary method to determine the complex pressure sensitivity of laboratory standard microphones is the pressure reciprocity calibration method specified in IEC 61094-2. Up to now, this method has been applied around the world almost exclusively to calibrate laboratory standard microphones by the manufacturer Brüel & Kjær. Consequently, only Brüel & Kjær microphones are currently used as reference standard for the sound pressure. In this paper, microphones of the type G.R.A.S. 40AU-1 were evaluated as an alternative and calibrated according to the standard. The results of comparison measurements are shown in shortened fashion. Further research questions are given.

Diffuse-field reciprocity calibration of half-inch laboratory standard microphones using simultaneous reverberation time measurement.

This paper presents a procedure for diffuse-field calibration of half-inch laboratory standard microphones by reciprocity using three microphones and a small reverberation chamber. The procedure consists of obtaining the impulse response of the system microphone-chamber-microphone and employing it to determine both the electrical transfer impedance between the microphones and the chamber's reverberation time (necessary to calculate the acoustic transfer impedance between the microphones). The electrical transfer impedance is calculated from the reverberant part of the obtained impulse response and the reverberation time is obtained by integrating the same impulse response. A comparison between results obtained by the proposed procedure (by the reciprocity method) and pressure field results corrected to diffuse-field (an usual alternative method) shows the accuracy of the proposal, while a comparison with other diffuse-field procedures using the reciprocity method shows that the measured frequency range is extended down, whereas similar performance is found in the common range.

Pressure reciprocity calibrations of laboratory standard microphones at the National Institute of Standards and Technology

The reciprocity technique has long served as a method for pressure calibration of microphones. It is a primary method, which determines microphone sensitivities from first principles and does not require a previously calibrated acoustic transfer standard. For calibrations of laboratory standard microphones, this method is standardized and utilized at national measurement institutes worldwide. Standard microphones calibrated by reciprocity are in turn used to calibrate additional microphones and sound calibrators, which apply known sound pressures to calibrate acoustical measuring devices and systems. Reciprocity calibrations done at the National Institute of Standards and Technology (NIST), which is the national measurement institute for the U.S., provide its customers with accurate results traceable to the International System of Units (SI). These customers and organizations that utilize their services perform large numbers of secondary and further calibrations and measurements concerned with hearing conservation and testing, aircraft noise, noise regulation enforcement, acoustical test and measurement equipment, and auditory research. An overview of the reciprocity technique is presented along with a few examples of how customers of the NIST acoustical calibration services make use of their calibrated devices and calibration data.

Pressure reciprocity calibration of a MEMS microphone.

This article reports the first use of the pressure reciprocity technique to calibrate a micro-electromechanical system (MEMS) microphone. This standardized primary calibration method is conventionally used to calibrate laboratory standard microphones. Results for the pressure reciprocity calibration of a MEMS microphone and two laboratory standard microphones are presented for the frequency range 100-10 000 Hz. Because the amplifier in the MEMS microphone package prevents reciprocal operation, this microphone was used only as a receiver of sound. A description of the procedure is presented along with checks of the measurement results and data regarding the uncertainties of these results.

Analysing the effects of phase sensitivity in low frequency primary microphone calibrations

Phase sensitivity data, in pressure reciprocity calibrations, has largely been overlooked due to the focus historically mainly only been on the modulus data. This study presents the low frequency variations that were observed between the modulus and phase sensitivities of LS2P microphones using multiple plane-wave couplers. It is shown that low frequency phase sensitivity data is more sensitive and is influenced more by the coupler and microphone geometries. Using plane-wave couplers with a longer cavity length, i.e. larger volumes, produces the best results. Differences has been observed between modulus and phase sensitivity data in the frequency range 1–100Hz, which brings attention to the relevance of also evaluating phase sensitivity data more closely in the context of primary pressure reciprocity calibrations and not only the modulus data. It is observed that complex sensitivity data, more specifically phase sensitivity data has more relevance than just that of determining the resonant frequency of Laboratory Standard condenser microphones. Measurement uncertainties relating to low frequency reproducibility are also presented and it is shown that the uncertainties related to the phase sensitivity low frequency reproducibility are more prevalent than for the same modulus data. Evaluation of low frequency phase sensitivity data has resulted in a 66% smaller phase uncertainty of measurement at 1Hz. It is concluded that smaller low frequency uncertainties can be obtained by using longer plane-wave couplers and that phase sensitivity data can be utilised as an analytical tool for improving low frequency measurement accuracy.

Calibration and characterization of MEMS microphones

In recent years, the increase in the number of smartphones led to a remarkable demand for low-cost microphones. This was met by the rapid development of MEMS (MicroElectroMechanical Systems) microphones, whose technology is becoming a promising perspective for future noise measurements based on new acoustic sensor networks. Nevertheless, current Standards do not provide proper calibration and test procedures for these microphones. In this work, calibration standard procedures have been adapted to characterize condenser MEMS microphones by comparison technique with laboratory standard microphones. Microphone parameters (sensitivity, frequency response, linearity, directivity, stability, and dynamic range) and changes of sensitivity with temperature (from -10 °C to + 50 °C) and humidity (from 25% to 90%) have been evaluated in a hemi-anechoic room and in an environmental chamber, respectively. These procedures open up the possibility to provide a robust and metrological characterization of MEMS microphones for noise measurements.

Electrical cross-talk subtraction for free-field reciprocity calibration of half-inch laboratory standard microphones.

This paper presents a procedure for free-field calibration of half-inch laboratory standard microphones by reciprocity that allows the identification and removal of the electrical cross-talk, multiple backscattering, reflections, noise, and all harmonic distortion components when measuring the electrical transfer impedance between the microphones. Initially, the difficulties of free-field calibration of condenser microphones by the reciprocity method are pointed out and the state of art is summarized. Following, the proposed procedure, which allows determining microphone sensitivities in the frequency range from 1 kHz to 40 kHz, is described: The cross-talk is removed by subtraction while multiple backscattering, reflections, noise and all harmonic distortion components are removed by a time-selective technique. Measurements and results are presented and discussed. A comparison between the results obtained by the proposed procedure and the pressure field calibration results corrected to free-field shows the efficiency and accuracy of the technique.

Report on key comparison COOMET.AUV.A-K5: pressure calibration of laboratory standard microphones in the frequency range 2 Hz to 10 kHz

This is the final report for regional key comparison COOMET.AUV.A-K5 on the pressure calibration of laboratory standard microphones in the frequency range from 2 Hz to 10 kHz. Two laboratories—Central Office of Measures (GUM)—the national metrology institute for Poland and the State Enterprise Scientific-Research Institute for Metrology of Measurement and Control Systems (DP NDI Systema)— the designated institute for acoustics in Ukraine took part in this comparison with the GUM as a pilot. One travelling type LS1P microphone was circulated to the participants and results in the form of regular calibration certificates were collected. The results of the DP NDI Systema obtained in this comparison were linked to the CCAUV.A-K5 key comparison through the joint participation of the GUM. The degrees of equivalence were computed for DP NDI Systema with respect to the CCAUV.A-K5 key comparison reference value.Main textTo reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCAUV, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Primary calibration in acoustics metrology

SI unit in acoustics is realized by the reciprocity calibrations of laboratory standard microphones in pressure field, free field and diffuse field. Calibrations in pressure field and in free field are already consolidated and the Inmetro already done them. Calibration in diffuse field is not yet consolidated, however, some national metrology institutes, including Inmetro, are conducting researches on this subject. This paper presents the reciprocity calibration, the results of Inmetro in recent key comparisons and the research that is being developed for the implementation of reciprocity calibration in diffuse field.