Tue26Sep20174:00 pmLewis 101
Colloquium: Vibrations, from Periodic Structures to the Human Face
Department of Mechanical Engineering
University of Mississippi
Vibrations: from Periodic Structures to the Human Face
This talk covers four different and yet connected subjects; Resonant Ultrasound Spectroscopy (RUS), vibration analysis of periodic structures,
and using facial vibrations in wearable computers. RUS is a technique to characterize the elastic and anelastic properties of materials. It is based on the measurements of the vibration eigenmodes of a sample with simple geometry such as a parallelepiped. In Laser RUS, the excitation part is done by a pulsed laser, generating thermoelastically excited ultrasonic pulse. In the detection side, a photorefractive interferometer is used to detect ultrasound. Measured eigenmodes along with eigenfrequencies reveal much information with regard to micro-structural state of the sample material. Novel techniques/problems in laser RUS is discussed in this section. In the second part, periodic structures are discussed. In periodic lattice structures, analysis of wave propagation to uncover dispersion relationships can be greatly simplified by invoking the Floquet-Bloch theorem. The accompanying Bloch formalism, which was first introduced for the study of quantum mechanics and has been borrowed in structural analysis, allows a system's degrees of freedom to be reduced to a small subset contained in a single unit cell. When this is combined with the finite element method, the result is a powerful framework for analyzing wave propagation and dispersion in complex media. In this section, among other things, the manner in which damping affects dispersion is talked about. In the next part, I talk about reciprocity in acoustics and how to break it; one way to break time reversal symmetry is to have a moving wave propagation medium. If the acoustic wave vector and the moving fluid velocity are collinear, we can use the wave vector shift caused by the fluid flow to break reciprocity. An alternative approach we have taken, is to use a fluid velocity field which enters the differential equation of the system as a cross product term with the wave vector. In the final part, bone conduction hearing is discussed; how it helps hearing and how it can be utilized for better communications in wearable technologies.