Certain physical characteristics such as apparent mass and stiffness influence the dynamic response of the head and thereby the degree of trauma suffered from impact with another body. These characteristics are a function of frequency and can be determined by mechanical impedance measurement techniques. A force generator was attached directly to the skull and the force input and resulting motion at the point of attachment were measured respectively by a force and acceleration transducer. The magnitude as well as phase angle between these two vectors were measured over the frequency range from 5 to 5,000 Hz.

A plot of the ratio of force and acceleration vs. frequency and phase angle vs. frequency on a nomograph reveal that both the apparent mass and stiffness of the head vary markedly from static values, and with location. Effective values of these physical properties at a given vibration frequency correspond to those arising during an impulse containing components at the same frequency. Results and relationship of this information to skull deformation and acceleration are shown and discussed.