A method for calibration of bone driver transducers to measure the mastoid impedance

When using bone vibrator transducers for clinical measurements, the transfer of energy from the bone driver depends on the impedance match between the driver and the load (human mastoid or otherwise) to which the driver will be applied. Current clinical calibration methods are incapable of quantifying this impedance mismatch, hence they fail to account for inter-subject variations of the energy transferred from the driver to the load. This study proposes a straightforward method for determining an absolute field calibration of a Radio Ear B71 bone driver, found by measuring the electrical input impedance of the transducer loaded by known masses. This absolute calibration is based upon a circuit model of the driver, describing it with three frequency-dependent parameters. Once these three parameters are known, measurements of the driver input voltage and current may be used to determine arbitrary mechanical load impedances (such as the in situ mastoid impedance), and thus the frequency dependence of the transmitted energy. The results of the proposed calibration method are validated by comparison with direct mechanical measurements using specialized equipment not available in the clinic, and a refined bone driver circuit model is proposed to better capture the observed behaviors.