Methods are developed for study of isotropic cubes via resonant ultrasound spectroscopy. To that end, mode structure maps are determined for freely vibrating isotropic cubes via finite element method over the full range of Poisson's ratio ν (−1 to +0.5). The fundamental torsional mode has the lowest frequency provided ν is between about −0.31 and +0.5. Experimental measurements for the mode structures of materials with Poisson's ratio +0.33, +0.3, +0.15, −0.15, and −0.72 are performed using

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... nant ultrasound spectroscopy and interpreted. Methods are developed to identify pertinent modes. The experimental results match well with the analysis with the exception of some splitting of some modes because of slight material anisotropy. The effects of slight imperfection of specimen shape on the first 10 modes are analyzed for various Poisson's ratios.