Reciprocity Calibration Of Microphones Research Articles

Characterization and Clinical Trials of the Prototype Ear Simulator Designed for Neonates Hearing Assessment

This article aims to describe the characterization of the new prototype ear simulator for neonates and demonstrate its utility in clinical measurements. Experimental evaluations have been performed in order to demonstrate consistency with the theoretical model in acoustic transfer impedance. Temperature and atmospheric pressure dependency measurements have been applied for verification of the stability of the ear simulator in variable environmental conditions. Thus, the main objective of this study is to outline the benefits of the new ear simulator in audiometric measurements for neonates. Acoustic transfer impedance of the ear simulator has been determined and compared with the theoretical model, and its environmental dependence has been studied between 100 Hz and 10,000 Hz. A modified method usually applied for reciprocity calibration of microphones has been used in measurements. Comparison of the new ear simulator with the IEC 60318-4 ear simulator and a \(2\,\hbox {cm}^{3}\) coupler has been presented. The ear simulator has been used to calibrate acoustics stimuli from an audiometer (Interacoustics AD226 with Otometrics insert earphone) and two otoacoustic emission devices (Otodynamics Otoport and Interacoustics Titan). The consistency of experimental evaluations in acoustic transfer impedance with the theoretical model has been confirmed. In clinical trials, an audiometer and two otoacoustic emission devices were investigated and a smaller standard deviation of 1.0 dB, 0.8 dB and 0.7 dB at 1 kHz, 2 kHz and 4 kHz, respectively, has been achieved compared to the old \(2\,\hbox {cm}^{3}\) coupler which has a 1.5 dB, 1.8 dB and 1.9 dB standard deviations at same frequencies. Average differences and standard deviations between the two couplers (IEC 60318-4 and the universal ear simulator) are \(11.4 \pm 0.3\,\hbox {dB}\) at 1 kHz, \(11.0 \pm 0.1\,\hbox {dB}\) at 2 kHz and \(13.3 \pm 0.2\,\hbox {dB}\) at 4 kHz. These differences represent the error incurred by using an adult ear simulator for neonates applications. The new universal ear simulator designed especially for hearing assessments of neonates has been characterized and used in clinical trials. The consistency between theoretical model and experimental measurements has been approved. Clinical trials showed that a more accurate calibration of OAE and audiometers for neonates would be possible. However, further applications for different age groups and broader clinical trials should be planned. Intercomparisons between different laboratories and clinics can maintain the comparability of hearing measurements and higher impact.

Reciprocity Measurements in Acoustical and Mechano-Acoustical Systems. Review of Theory and Applications

In 1873 Lord Rayleigh defined the General Reciprocity Theorem for stable, finite, lumped, passive, linear dynamical systems which only contain reversible (bilateral) elements. In spite of Rayleigh's authority, there have been a number of papers expressing doubts about the validity of the theorem for certain acoustical and mechano-acoustical systems. It can be shown however, that these papers are either based on too primitive experiments or on too simple theoretical models. On the other hand there are many publications showing the validity of the theorem. It can be concluded that Rayleigh's theorem is correct. Flow disturbs reciprocity because it introduces non-reversible elements. Nevertheless, in many systems with flow, the disturbing effect is limited and reciprocity can still be applied. The most straightforward application of reciprocity is the measurement of transfer functions. Second in order, because it requires two measurements instead of one, is the measurement of acoustical or mechanical source strength by reciprocal substitution. Third in order, because it needs three measurements, is the reciprocity calibration of microphones and hydrophones. Until 1970, only the latter application was known. In the period between 1970 and 1988 the author of this paper and his colleagues developed a number of methods for the reciprocal measurement of transfer functions and source strength in mechano-acoustical and acoustical systems. It was shown that the reciprocal measurement of transfer functions and of source strength has often great advantages over the direct alternatives. Since 1985, others joined the efforts. Particularly J. W. Verheij, F. J. Fahy and I. L. Ver have done very much for the further development and dissemination of the methods. Nowadays they are widely applied in the automotive industry and in the shipbuilding industry, where they are used for transfer path analysis, data gathering for prediction methods, source identification, source characterisation and for radiation measurements. The methods are also applied to aircraft, trains and buildings, but the dissemination of the methods for those systems is less than for cars, trucks and ships. Further applications concern the development of quiet gearboxes, the development of quiet fans, the prediction of the effects of sonic booms and the design of musical instruments.

Free‐field reciprocity calibration of microphones in ultrasonic frequency range

Exposure to airborne low‐frequency ultrasound occurs in many industrial applications such as cleaning, welding plastics, measuring distances in buildings, and by using consumer devices such as camera rangefinders, automatic door openers, parametric ultrasound loudspeakers, etc. Exposure to ultrasound in air may be dangerous to the hearing and may have negative bio‐effects on humans. Care should be taken in its use. In order to establish appropriate limits and to measure the output of ultrasound devices, there is a need to develop a sound‐pressure standard in the frequency range from 20 kHz to about 160 kHz. A calibration project for quarter‐inch microphones by the reciprocity method has been started in the PTB, and a new automated measurement setup for free‐field reciprocity calibration of quarter‐inch microphones has been established. A procedure for a free‐field reciprocity calibration of quarter‐inch microphones in the frequency range 20 to 160 kHz is described. First results of free‐field calibration of quarter‐inch microphones are presented, giving the sensitivities and the repeatability of results. The effects of reflections and cross talk on the accuracy of the measurements will be explained.

Reciprocity Calibration of Microphones in High Ambient Pressure

Research concerning man's ability to speak and hear within ambient conditions of many times normal atmospheric pressure poses special difficulties in calibrating psychoacoustic equipment. In this experiment, a system for obtaining reciprocity calibrations within hyperbaric environments was employed. By means of the reciprocity system, microphone sensitivities were calculated from measures taken with a piezoelectric microphone. Frequencies were 0.1 6 kHz, and conditions of ambient pressure within a helium-filled hyperbaric chamber were equivalent to pressures encountered from surface to 600-ft submersion in sea water (1–19 ata absolute). The results indicate that, at frequencies below 1 kHz, increases in ambient pressure from surface to depths of 600 ft were accompanied by consistent decreases in microphone sensitivity. At 2 kHz, sensitivity did not continue to decrease for pressures equivalent to submergence from 300 to 600 ft. Resultant corrections to the calibrated microphone for increases in ambient pressure permit accurate measures to be made of stimuli for psychoacoustic experiments, and precise acoustic analyses can be made of speech produced under hyperbaric conditions.

Microphone-Reciprocity Calibration

A simplified technique of performing a closed-coupler (cylindrical cavity) reciprocity calibration of microphones is discussed All three transducers needed in the calibration procedure remain as parts of the coupler throughout the measurement. A multiple-function switch is used to connect the circuits for the required operations in sequence, without physically interchanging transducers. The same switch operates an analog computer, which uses a logarithmic potentiometer to solve the necessary calculations. The auxiliary transducer is a piezoelectric ceramic ring, which makes up the cylindrical wall of the cavity; thus, difficulties associated with having all three transducers in the coupler at the same time are avoided. Considerations involved in using this piezoelectric ring are discussed. Additional features of the calibration technique are (1) use of a capacitor in the measurement of the driving current of the reciprocal transducer, and (2) use of a resistive-attenuator-insert technique in the measurement of open-circuit voltage ratios. The calibrator is designed for calibration of a front-damped piezoelectric-ceramic microphone, and the problems resulting from the front-damping are discussed.

Reciprocity Calibration of Microphones in a Diffuse Sound Field

Microphones can be calibrated by a primary technique in a diffuse sound field. The formula for the diffuse-field voltage response is derived from the well-known relationship for a reciprocal transducer in free space. It is shown that the reciprocity parameter for a diffuse-sound field follows from that for free space by replacing the distance by the “diffuse-field distance” of a point source. This distance depends only on the total absorption in the reverberation room. The experimental procedure is described in some detail and the results of the diffuse-field calibration of a Western Electric 640 AA condenser microphone are presented.

Reciprocity Calibration of Microphones Using a Pulse Technique

Acoustical calibration of a microphone may be made conveniently either by free-field calibration or by calibration using a small cavity. Free-field calibrations are difficult because of the necessity for establishing a space which allows for propagation of a single progressive wave, without any interference. A progressive wave may be set up in a room which will be free from interference at a given point until sound energy arrives by reflection from the nearest wall surface. By sampling the output from a microphone during this period of time, a signal may be obtained which determines the microphone response to a free progressive wave. Periodic repetition of this process gives a series of pulses from the microphone which may be used to obtain a reciprocity calibration.