Index of content:
Volume 125, Issue 2, February 2009
- STRUCTURAL ACOUSTICS AND VIBRATION 
125(2009); http://dx.doi.org/10.1121/1.3058632View Description Hide Description
In a family of drums used in the Indian subcontinent, the circular drum head is made of material of nonuniform density. Remarkably, and in contrast to a circular membrane of uniform density, the low eigenmodes of the nonuniform membrane are harmonic. In this work the drum head is modeled as a nonuniform membrane whose density varies smoothly between two prescribed values. The eigenmodes and eigenvalues of the drum head are obtained using a high-resolution numerical method. The mathematical model and the numerical method are able to handle both concentric and eccentric nonuniformities, which correspond, respectively, to the dayan and the bayandrums. For a suitable choice of parameters, which are found by optimizing the harmonicity of the drum, the eigenspectra obtained from the model are in excellent agreement with experiment. The model and the numerical method should find application in numerical sound synthesis.
125(2009); http://dx.doi.org/10.1121/1.3050288View Description Hide Description
Acoustic metafluids are defined as the class of fluids that allow one domain of fluid to acoustically mimic another, as exemplified by acoustic cloaks. It is shown that the most general class of acoustic metafluids are materials with anisotropic inertia and the elastic properties of what are known as pentamode materials. The derivation uses the notion of finite deformation to define the transformation of one region to another. The main result is found by considering energy density in the original and transformed regions. Properties of acoustic metafluids are discussed, and general conditions are found which ensure that the mapped fluid has isotropic inertia, which potentially opens up the possibility of achieving broadband cloaking.