The results of a study of the three-dimensional vibration of two dry human skulls in response to harmonic excitation are presented. The vibratory response exhibits three distinct types of motion across the range of audible frequencies. At low frequencies below 1000 Hz, whole-head quasi-rigid motion is seen. At the middle frequencies between 1000 and 6000 Hz, the motion exhibits a series of increasingly complex modal patterns. Above 6000 Hz, the response is wavelike and clear wavefronts can be distinguished in the vibration data. In this regime the relationship between wavelength and frequency is calculated and compared to a number of theories of skull vibration that have been proposed.

Abstract

The results of an experimental study of the vibratory response of the dry human skull at acoustic frequencies (250 Hz–16 kHz) are presented. In three dry skulls, the motor from a clinical bone vibrator, the BAHA Divino, was used to excite vibrations, and the amplitude and phase of the response were measured with a Polytech PSV-300 scanning laser Doppler vibrometer. Data were acquired at 2200 points covering the entire skull surface at ∼5 mm resolution. The experiment followed a stepped-sine paradigm with excitation frequencies spaced linearly at 200 Hz. The experiments show that the dry skull undergoes quasi-rigid motion at low frequencies, which breaks up into modal vibration patterns above 1000 Hz. At frequencies above 8000 Hz, clear standing wave patterns can be seen with wavefronts radiating out from the excitation location. The data are consistent with the response being primarily plate-bending motion. Progress of reproducing the data with a finite element model will also be reported. Geometry for the model was acquired using a high-resolution CT scan imported into HyperMesh® and the simulation was performed in LS-DYNA® hydrocode. [Work supported by the Atlantic Canada Opportunity Agency’s (ACOA) Atlantic Innovation Fund and the Natural Sciences and Engineering Research Council (NSERC).]

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.