For an isotropic material, Shear Modulus is calculated from Young's Modulus and Poisson's Ratio.

For Young's Modulus =Â 9.8E9 Pa and Poisson's Ratio of 0.44, Shear Modulus is 3.4E9 Pa.

The orthotropic material you show above has a Shear Modulus about 1/3 of an isotropic material.

The table below shows how the frequency of Mode 8 changes as the Shear Modulus is reduced.

The shell element mode 8 is affected while the solid element mode 8 is not affected by the Shear Modulus.

Mode 7 is the bending mode

while Mode 8 is a twisting mode that creates a lot of shear in the material.

The change in Mode 8 natural frequency with Shear Modulus provides confirmation that the shell elements areÂ using this value appropriately, and that a single layer of solid elements are not. Shell elements are the right choice for a Modal Analysis of a thin structure with orthotropic properties that include lower values of Shear Modulus when there is a significant shear mode present.

Your musical instrument is not floating in space, so a more appropriate boundary condition might be for the rectangular plate to be either simply supported or fixed on the edges. The best boundary condition for the model is the one that represents the experimental setup.