Force sensitivity calibration of artificial mastoids

The Standards and Calibration Laboratory (SCL) has set up a system for calibration of artificial mastoids to meet the growing demand for calibration of bone-conduction hearing aids and audiometers in the audiology industry of Hong Kong. An artificial mastoid (or mechanical coupler, as specified in IEC 60318-6) is a device that mimics the mechanical characteristics of the human mastoid (one of the bones of the skull behind the ear). It is basically composed of a dome-shaped structure and a force transducer. This paper mainly illustrates the method for measuring the force sensitivity level (in unit of dB re 1 pC/N) of an artificial mastoid in the frequency range of 125 Hz to 8 kHz. The calibration system has been automated to meet the demand for artificial mastoid calibration services in Hong Kong. This paper describes (i) the method of determining the force sensitivity level of an artificial mastoid, (ii) the measurement model and uncertainty evaluation, and (iii) the measurement results and details of the calibration system.