NPL Acoustics Group provides calibration services for ear simulator sytems, with traceability to national standards. This spans all types of artificial ear / reference coupler and associated reference earphones ('Air Conduction' audiometric test systems), as well as artificial mastoids (sometimes called mechanical couplers) and associated reference bone vibrators ('Bone Conduction' audiometric test systems). These instruments normally form part of a system used to calibrate audiometers, which will be used to test hearing thresholds.

Requirements for the periodic verification of the performance of an audiometer are specified in IEC 60645-1 and ISO 8253-1. Clearly, if hearing impairment is to be characterised and quantified consistently then it is necessary to ensure correct delivery of stimulus to the subject; measurements that lack traceability cannot be considered reliable, even if some other non-traceable form of 'calibration' has been applied, and could lead to incorrect diagnosis.

Calibration of Ear Simulator Systems: Air Conduction

NPL's 'Air Conduction' audiometric calibrations are carried out using calibrated devices that are traceable to the national standards maintained at NPL. Artificial ear / reference couplers are calibrated by comparison with NPL's reference microphones, themselves calibrated to the best measurement capability available at NPL.

ISO 389-1 specifies that either the IEC reference coupler or artificial ear be used for the calibration of supra aural earphones (depending on their type). These devices contain a measurement microphone system, that is in turn typically connected to a device such as a sound level meter or analyser. These instruments can then be used to read the sound pressure level at the microphone, and perform frequency analysis.

For the convenience of the user, the calibration service offered by NPL usually considers the complete system (see below), which is calibrated by determining the response of the system when a known sound pressure level is applied. The calibration process compares the response of the customer's system against an NPL calibrated reference laboratory standard microphone; covering eleven audiometric frequencies in the range 125 Hz to 8 kHz. The overall measurement uncertainty of calibration is typically 0.2 dB (at a confidence of approximately 95%). For artificial ears fitted with suitable microphones, the calibration can be extended up to 16 kHz.

NPL recommends that, as part of having an artificial ear / reference coupler system calibrated, a pair of reference earphones be measured on the device immediately after calibration. The user can then repeat these measurements periodically, in order to monitor the stability of the system. In this measurement, each earphone is driven with a known voltage and the resultant output reading from the system recorded. Measurements are made at the same frequencies used for the calibration of reference coupler or artificial ear systems and reported together with a typical standard deviation.

Equipment Required for NPL Calibration of an Air-Conduction System

A complete air conduction system should be comprised of:

either

• IEC acoustic reference coupler complying with BS EN 60318-3 (IEC 60318-3)
• Microphone; WS1P adapted to look like a LS1P (ie. grid replaced by adaptor ring) complying with BS EN 60318-3

or

• IEC wide-band artificial ear complying with BS EN 60318-1 (IEC 60318-1)
• Microphone; WS2P (ie. including grid) complying with BS EN 60318-1 (IEC 60318-1)

Plus:

• Sound calibrator / pistonphone (including appropriate adaptor)
• Type 1 sound level meter, analyser or similar measuring device complying with BS EN 61672
• Octave or 1/3-octave filter set complying with BS EN 61260
• Appropriate pre-amplifier, connecting leads and adaptors

Optional:

• Reference earphones and connecting leads
• Loading arm or loading mass

Calibration of Ear Simulator Systems: Bone Conduction

NPL's 'Bone Conduction' audiometric calibrations are carried out using calibrated devices, to ensure direct traceability. As part of calibration, artificial mastoids (mechanical couplers) are both verified to ensure impedance complies with IEC 60318-6, as well as being calibrated for their force sensitivity level.

The bone conduction channel of an audiometer is calibrated using a mechanical coupler, typically connected to a sound level meter. For the convenience of the user, the calibration service offered by NPL considers the complete system (see below), which is calibrated by applying a known vibratory force to the coupler's dome and noting the system response. This calibration covers the eight common audiometric frequencies in the range 250 Hz - 4 kHz. The overall uncertainty of calibration at a confidence of approximately 95% is typically 0.4 dB. Extending the calibration to 8 kHz is also possible, though extended mechanical coupler calibrations have caveats and should be discussed with NPL first.

NPL recommends that a mechanical coupler calibration be accompanied by the measurement of two reference bone vibrators. Each bone vibrator is driven with a known voltage and the resultant output reading from the system is recorded. The user can then repeat these measurements periodically, in order to monitor the stability of the system. Measurements are made at the same eight frequencies used for the calibration of the mechanical coupler and reported together with a typical standard deviation.

Equipment Required for NPL Calibration of a Bone-Conduction System

A complete bone conduction system should be comprised of:

Required:

• Type 1 sound level meter, analyser or similar measuring device complying with BS EN 61672-1 (IEC 61672-1)
• Octave or 1/3-octave filter set complying with BS EN 61260 (IEC 61260)
• IEC mechanical coupler complying with BS EN 60318 (IEC 60318)
• Appropriate connecting leads and adaptors

Optional:

• Reference bone vibrators and connecting leads
• Loading arm or loading mass

See also

Acoustics Technical Guides: Audiometric Calibration and Standards.

Contact

Customer Service tel: +44 20 8943 8631
E-mail: acoustics_enquiries@npl.co.uk