Poisson solid ($\lambda =\mu ,\nu =1/4,{\alpha }^2=3{\beta }^2$)

An isotropic linear elasticity material with equal positive Lamé parameters has Poisson's ratio $1/4$ and the displayed compressional/shear speed ratio. This simplifying model fixes the relative amplitudes in the tensile-crack radiation pattern; it does not describe every stable isotropic solid.