Mechanical Impedance of the Forehead and Mastoid

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Abstract
A method was developed for determining the resistive and reactive components of the impedance of the human head and mastoid, or of other high mechanical impedances, using a special direct-recording measuring system that records both force and motion. Impedances presented to driving tips of 12.5 and 20 mm diameter were measured over the audio-frequency range from 40 to 10,000 cps. The effects of static force coupling the head to the driver were compared for 500-and 1000-g coupling forces. The driving system consisted of a tubular section of polarized barium titanate, rigidly clamped at one end to a massive wall. Force and motion were measured close to the point of contact with the load, to minimize errors arising from flexure in the driver or between the sensing elements. The system was calibrated by loading the driving tip with known masses. Over a large part of the audio-frequency range, the predominant characteristic of both head and mastoid impedance is a stiffness reactance, with some damping. In the high-frequency range, a small mass reactance parameter appears. The mass load due to the whole head influences the impedance only at the low-frequency extreme of the measurement range.

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