Three-dimensional laser Doppler vibrometry of the dry human skull

The results of two experimental studies of the vibratory response of dry human skulls at acoustic frequencies (100 Hz-20 kHz) are presented. In both experiments the motor from a commercial bone vibrator, the Baha Divino{trade mark, serif}, was used to excite vibrations in dry human skulls. The first experiment used stepped sine wave excitation to drive vibrations in the skull bone, and the amplitude and phase of the response were recorded with a scanning laser Doppler vibrometer (LDV) at 2200 points over the skull. Three distinct kinds of vibration were seen over the measured frequency range: 1) quasi-rigid whole-head motion at frequencies below 1000 Hz, 2) modal vibration above 1000 Hz and 3) wave propagation above 8000 Hz. The second experiment used swept sine wave excitation and measured the vibratory response at five locations with a high frequency resolution in order to identify resonant frequencies. At the high end of the frequency range, the dependency of the mode number on frequency was found to follow a square root dependency, consistent with vibrations being flexural waves. This result is supported by analytical and finite element models of a sphere with similar geometric and material properties.