  In the literature, to extract the dispersion relation of low-frequency
quasinormal modes in both diffusive and shear channels, it is a customary
recipe to assume firstly $\omega\sim\mathcal{O}$ to solve the equation of
motion and finally $\omega\sim\mathcal{O}(q^2)$ when applying the Dirichlet
boundary condition. The two assumptions appear confusing though the recipe
usually gives the same result as that from other channels or from the Kubo
formula. We refine the recipe by assuming $\omega\sim\mathcal{O}(q^2)$ from the
beginning to the end, and demonstrate it in the diffusive channel of the
Schwarzschild black brane and the shear channel of the Gauss-Bonnet black
brane.
