Content area
Full Text
Abstract
A degree of judgement is required when applying the British Standard associated with the calibration and performance of audiometers, BS EN 60645-1 (IEC 645-1). Consequently there is potential for variation of practice between calibration agencies. This note addresses issues and makes suggestions relating to the measurement of distortion, on-off effect, cross talk, linearity, rise/fall time and to the positioning of the earphone on an artificial ear. The degree of precision appropriate to each measurement is suggested. A warning is given of the audiological consequences of excessive distortion produced by normally functioning audiometers.
Key words: audiometer, calibration, distortion, standards
Introduction
The British Standard describing audiometer specification and performance, BS EN 60645-1 (BSI, 1995), is highly detailed in many respects, yet there are a few areas of uncertainty that arise as soon as an audiometer needs calibrating. Sometimes the uncertainty is in the particular conditions most appropriate for a measurement. Unless stated to the contrary, the performance specification in the standard is assumed to apply regardless of audiometer setting or configuration, yet it is unreasonable to make measurements under every possible combination of settings. For example, frequency accuracy is measured at only one intensity using one earphone and it is assumed that changing intensity or earphone will have no effect on frequency. For many calibration parameters, it makes sense to perform the measurement under conditions most likely to expose a problem - the worst-case scenario. The following interpretative comments are offered in the hope that they will be helpful to students in audiology and serve as a point of reference or discussion for the BSA Standards Forum, calibration agencies, audiometer manufacturers and perhaps even standards committees. However, these are personal interpretations and therefore carry no official endorsement.
Distortion
This applies to both air- and bone-conduction transducers. BS EN 60645-1 (BSI, 1995) gives the maximum permissible second, third and total harmonic distortion (THD) as per cent RMS amplitude at various intensities and frequencies:
* Air-conduction (second and third harmonics): 2%.
* Bone-conduction second harmonic: 5%.
* Bone-conduction third harmonic: 2%.
For convenience of measurement with a sound level meter (SLM), the quoted levels of 2% and 5% distortion are equivalent to an indicated -34 dB and -26 dB relative to the fundamental, respectively.
The standard...