Measurements of the vibrational response of a spherical aluminum shell subject to changes in the interior pressure clearly demonstrate that resonance frequencies shift higher as the pressure is increased. The frequency shift appears to be smaller for longitudinal modes than for bending wave modes. The magnitude of frequency shift is comparable to analytical predictions made for thin cylindrical shells. Changes in the amplitudes of resonance peaks are also observed. A possible application of this result is a method for noninvasively monitoring pressure changes inside sealed containers, including intracranial pressure in humans.

A possible method for noninvasively monitoring changes in intracranial pressure is to measure changes in skull resonance frequencies. Recent measurements of the vibrational response of a spherical aluminum shell clearly demonstrate that resonance frequencies shift higher as the internal pressure is increased [Piacsek et al, J. Acous. Soc. Am, 131, EL506-510 (2012)]. The frequency shift is approximately linear with the applied pressure, regardless of whether the shell is filled with air or water, and circumferential modes exhibit larger resonance shift than longitudinal modes. A computational model of a fluid-filled thin shell subject to acoustic stimulation was developed using the COMSOL multi-physics software to investigate the role of shell material and geometry in resonance shifts. The model predicts frequency shifts comparable to those observed in the spherical aluminum shell. Preliminary computational results for a spherical shell made of bone-like material, as well as for asymmetric and nonuniform shells, will be presented.

A finite-element model of the vibrational response of fluid-filled shells with arbitrary shape and composition has been developed using the COMSOL multi-physics modeling package. The user can specify the properties of the fluid inside the shell including the static pressure. The shell is surrounded by air, which is enclosed by a perfectly matched layer boundary, and an acoustic source is positioned just outside the shell. The frequency response of the shell due to a swept sine acoustic excitation can be recorded at multiple locations. Model results for a spherical aluminum shell filled with water at different static pressures are compared with experiment. Results are also shown for a shell with geometry and material properties similar to a human skull. The goal is to apply this model to predict vibrational response due to changes in intracranial pressure. [Work supported by Central Washington University Science Honors Program.]