  Execution of Resonant Ultrasound Spectroscopy (RUS) for accurate measurement
of elastic constants lies primarily on a perfect matching in the calculated and
measured mode frequencies of free vibration. Calculation of these frequencies
require estimated values of the elastic constants of the material under study,
and one has to depend on other experiments for these data. The present work
proposes and demonstrates an alternative to derive initial guess values of
essential parameters for an isotropic and homogeneous material from the
acquired RUS spectra itself. Specimen samples are taken in the shape of
rectangular parallelepiped having nearly same cross-sectional dimension but
with different lengths. For particular compressional (shear) mode corresponding
to length l, the frequency fis inversely proportional to l. The slope m of f
versus 1/l plot equals to half of the compressional (shear) velocity and this
in turn gives an estimate of c_11 (c_44). With these parameters as the input
guess parameters, RUS fitting method is executed to find out the best fit
results. Elastic constants of commercially available specimens of aluminium,
copper, lead, steel and brass are measured in this technique assuming
macroscopic homogeneity. Results show good agreement with available literature
values.
