Richard Barham

Calibration of a micromachined particle velocity microphone in a standing wave tube using a LDA photon-correlation technique

In this paper, a new method of calibrating an acoustic particle velocity sensor using laser Doppler anemometry (LDA) is discussed. The results were compared and were in good agreement with the results obtained by conventional methods, where the sensitivity of the microflown is obtained with the use of a reference microphone and a standing wave tube. The LDA signal generated by the acoustic particle motion was analysed using the photon-correlation method, where the signal is considered to consist of a series of discrete photon events. The photon-correlation system is used to measure particle velocity amplitude next to the microflown particle velocity sensor in a standing wave. Measurements are performed for frequencies between 250 Hz and 4 kHz and velocities between 5 mm s-1 and 25 mm s-1 (root-mean-square (rms) values) which are equivalent to sound fields of 100 and 114 dB SPL in free field. From the output voltage of the probe microflown and the LDA-derived particle velocity in a standing wave, the sensitivity of the microflown is obtained. The two different calibration methods are in good agreement showing a discrepancy of 1 dB for the frequency range of 250 Hz–4 kHz.

The application of the NPL laser pistonphone to the international comparison of measurement microphones

NPL has chosen to use the laser pistonphone as the basis for some of its measurements for international key comparison CCAUV.A-K2. The other laboratories taking part in the key comparison have all used the reciprocity technique for their low-frequency measurements, thus the use of a laser pistonphone allows the verification (or otherwise) of this technique at low frequencies. Since the use of this device for the calibration of microphones is not currently included in international standards, this paper describes this calibration method and gives a full account of the associated measurement uncertainty. While the NPL laser pistonphone has existed since the 1970s, its true value to the international community is only now being revealed.This paper also provides the necessary background information to support the key comparison data when they are eventually published.

Final report on key comparison CCAUV.A-K5: pressure calibration of laboratory standard microphones in the frequency range 2 Hz to 10 kHz

This document and the accompanying spreadsheets constitute the final report for key comparison CCAUV.A-K5 on the pressure calibration of laboratory standard microphones in the frequency range from 2 Hz to 10 kHz. Twelve national measurement institutes took part in the key comparison and the National Physical Laboratory piloted the project. Two laboratory standard microphones IEC type LS1P were circulated to the participants and results in the form of regular calibration certificates were collected throughout the project. One of the microphones was subsequently deemed to have compromised stability for the purpose of deriving a reference value. Consequently the key comparison reference value (KCRV) has been made based on the weighted mean results for sensitivity level and for sensitivity phase from just one of the microphones. Corresponding degrees of equivalence (DoEs) have also been calculated and are presented. Main text. 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 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).

A new 3D finite element model of the IEC 60318-1 artificial ear

The artificial ear specified in IEC 60318-1 is used for the measurement of headphones and has been designed to present an acoustic load equivalent to that of normal human ears. In this respect it is specified in terms of an acoustical impedance, and modelled by a lumped parameter approach. However, this has some inherent frequency limitations and becomes less valid as the acoustic wavelength approaches the characteristic dimensions within the device. In addition, when sound propagates through structures such as narrow tubes, annular slits or over sharp corners, noticeable thermal and viscous effects take place causing further departure from the lumped parameter model. A new numerical model has therefore been developed, which gives proper consideration to the aforementioned effects. Both kinds of losses can be simulated by means of the LMS Virtual Lab acoustic software which facilitates finite and boundary element modelling of the whole artificial ear. A full 3D model of the artificial ear has therefore been developed based on key dimensional data found in IEC 60318-1. The model has been used to calculate the acoustical impedance, and the results compared with the corresponding data determined from the lumped parameter model. The numerical simulation of the artificial ear has been shown to provide realistic results, and is a powerful tool for developing a detailed understanding of the device. It is also proving valuable in the revision of IEC 60318-1 that is currently in progress.

Report on key comparison CCAUV.A-K1

This is the final report for key comparison CCAUV.A-K1 on the pressure calibration of laboratory standard microphones in the frequency range from 125 Hz to 8 kHz. Twelve national measurement laboratories 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 key comparison reference value (KCRV) has been calculated using the unweighted mean value of the results. Deviations from this value are below 0.05 dB at all frequencies. A frequency of 250 Hz has been chosen to illustrate the degrees of equivalence with the KCRV. In all but one case, the degree of equivalence is smaller than the associated uncertainty. Main text. 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 kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCAUV, according to the provisions of the Mutual Recognition Arrangement (MRA).

Secondary pressure calibration of measurement microphones

Recent developments are presented in the practical realization of IEC 61094-5 on the pressure calibration of working standard microphones. Particular emphasis is placed on the simultaneous comparison calibration approach, implemented in an open sound field. Limiting factors as well as those having a significant influence on the determination of the pressure sensitivity are discussed. These include the separation between the microphones, the influence of the test environment and location of the sound source, and issues arising from geometric and electroacoustic dissimilarities between the reference microphone and the microphone under test. Finally, recommendations are given for aspects to be considered in a future revision of IEC 61094-5, including contributions to the measurement uncertainty arising from the matters discussed.

A new 3D finite element model of the IEC 60318-1 artificial ear: II. Experimental and numerical validation

In part I, the feasibility of using three-dimensional (3D) finite elements (FEs) to model the acoustic behaviour of the IEC 60318-1 artificial ear was studied and the numerical approach compared with classical lumped elements modelling. It was shown that by using a more complex acoustic model that took account of thermo-viscous effects, geometric shapes and dimensions, it was possible to develop a realistic model. This model then had clear advantages in comparison with the models based on equivalent circuits using lumped parameters. In fact results from FE modelling produce a better understanding about the physical phenomena produced inside ear simulator couplers, facilitating spatial and temporal visualization of the sound fields produced.The objective of this study (part II) is to extend the investigation by validating the numerical calculations against measurements on an ear simulator conforming to IEC 60318-1. For this purpose, an appropriate commercially available device is taken and a complete 3D FE model developed for it. The numerical model is based on key dimensional data obtained with a non-destructive x-ray inspection technique. Measurements of the acoustic transfer impedance have been carried out on the same device at a national measurement institute using the method embodied in IEC 60318-1. Having accounted for the actual device dimensions, the thermo-viscous effects inside narrow slots and holes and environmental conditions, the results of the numerical modelling were found to be in good agreement with the measured values.

An alternative approach to the measurement of the acoustic transfer impedance of the IEC 60318-1 ear simulator

Ear simulators are important measurement devices for characterizing the performance and acoustic output of earphones and other transducers designed to be coupled to human ears. For best practice in the use of these devices it is important that the acoustic transfer impedance is periodically tested against published specification. This practice has been facilitated by IEC 60318-1 : 2009, where a test method has been specified. This paper reports an alternative method offering simplifications in concept and implementation, and leads to improved measurement uncertainty. A comparison of impedance measurements on a sample of ear simulators shows that the two methods produce equivalent results, and that the proposed method offers a viable alternative to that given in IEC 60318-1.

The calibration of a prototype occluded ear simulator designed for neonatal hearing assessment applications

An innovative family of ear simulators has been conceived for the calibration and traceability of audiometric equipment. Each device within the family has been designed for a particular key age group, covering neonates through to adults. The age-specific ear simulators are intended to improve the quality of hearing assessment measurements for all test subject age groups, and will be proposed as the next generation of standardised ear simulators for audiometric applications. The family of ear simulators shares a common design and modeling approach, and the first prototype devices for neonatal applications have been manufactured. The objectives of this study were to develop calibration methods, verify conformance to the design goals, demonstrate that the device is capable of being calibrated reliably, and show that its performance is ultimately suitable for international standardisation and eventual adoption into clinical practices. Four national measurement institutes took part in a round-robin calibration comparison and an analysis of the results showed that these objectives were achieved.

Practical considerations for the application of ear simulators in the calibration of audiometers in the extended high frequency region

Abstract Objective: Calibration service providers for audiometric equipment often encounter impracticalities in fully implementing the International Electrotechnical Commission (IEC) guidelines for the extended high frequency region. This report evaluates some of the work-around solutions sometimes employed in practice and the implications these have for audiometer calibration results and uncertainties. Design: The impact of using four different microphone configurations on the ear simulator calibration in the frequency range 125 Hz to 20 kHz, and especially in the extended high frequency range from 10 kHz to 20 kHz, was investigated, at a range of temperatures. Results: Variations in the response of the ear simulator of up to 6 dB were observed with the different microphone configurations. In addition, using the microphone without its protection grid produced a dip in the high frequency response of approximately 15 dB. Conclusion: While deviation from the practices required in IEC standards is not recommended, replacing the microphone protection grid with a specially fabricated collar (essentially a grid with the top removed) was found to constrain deviations in response to within ±2 dB. It was also concluded that simply removing the microphone protection grid resulted in a wholly unsatisfactory performance.