Driving point impedance characteristics of the head

Abstract

The mechanical impedance of the human and monkey (Macaca mulatta) head was determined over the frequency range 30-5000 Hz. Miniature accelerometers and pressure transducers were placed in the brain to measure its response to vibration at constant g-levels and variable frequency. The maximum acceleration studied was 20 g s. In vitro experiments on a fresh human cadaver and in vivo andin vitro experiments on monkeys were performed. The effect of varying blood pressure was investigated as well as the contribution to the mechanical impedance of the scalp, skull and brain.A linear two-degree-of-freedom model that summarizes the results with acceptable accuracy is presented. Certain non-linear responses were observed for various input accelerations. No significant effect on impedance due to time after death was found for times up to five hours.The implantable accelerometer and pressure transducer experiments indicated that the brain is very nearly critically damped. Raising the blood pressure was shown to stiffen the brain, causing the resonance frequency of the head to increase.