Mechanical Impedance of the Forehead and Mastoid

Abstract

A method has been 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 have been measured over the audio-frequency range from 40 to 10 000 cps. The effects of static force coupling the head to the driver have been compared for 500- and 1000-g coupling forces. 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. 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 lead, 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.

A Miniaturized Artificial Mastoid Using a Skull Simulator

“…11, the difference in output from a Radioear B-71 can vary several dB for a minor difference in mechanical load impedance level of the artificial mastoid. It is also well known that the intersubject variation in mechanical skull impedance is fairly large (Corliss & Koidan, 1955;Flottorp & Solberg, 1976;Håkansson et al, 1986), and the output from a Radioear B-71 can vary substantially on a subject compared to the calibrated values. One way to make a bone-conduction transducer more insensitive to variation in mechanical impedance is to incorporate damping in the transducer and not utilize the shell resonances.…”

“…In previous measurements of the mechanical impedance of the human mastoid, e.g. Corliss & Koidan (1955), Flottorp & Solberg (1976) and Håkansson et al (1986), it has been stated that, for frequencies between 100 Hz and 2 kHz, the skull is stiffness-controlled and, for frequencies between 3 and 10 kHz, it is basically masscontrolled. Also, the phase of the mechanical impedance of the Brüel & Kjaer type 4930 artificial mastoid is in accordance with this (Brüel & Kjaer, 1987).…”

The Biological Effects of Vibration

“…-Ih connection with -it.,mius in tOe use of vibrating hand tools several ei assurements have been made of vibration tra•nsiission from hand to ari and body (20,40,41). The impedance nrcosured on a hlnd grip fir a sree'ific condition reprcsentativw of hil1d tool use is presented in figure 10.…”

“…Onset rates endured by various subjects are therefore summarized in separate graphs for the different directions (figs. 35, 37,39,41). Caution in applying the curves presented must again be siressed, since they are based on well designed body supports, minimum slack of the harnesses, heavy seat construction, young, healthy volunteer subjects, and subjects expecting the impact exposure.…”

Recent Advances in the Measurement of Structural Impedance

“…2 The force which must be subtracted from the force channel equals Wxa, or a constant times the acceleration. It is therefore only necessary to subtract electronically from the force signal an acceleration signal of a properly adjusted constant amplitude.…”