Anharmonic interaction of low-frequency transverse waves in crystals

We evaluate the relaxation time of a normal process of three-phonon of the type T + T = L for low frequency by using an approximation based on the continuum approximation. We find that the relaxation time satisfies Herring's rule. We also compare the relative strengths among three different types of normal processes, T + T = L, L + T = L and T + L = L for a typical case.