Normal sound in gases is a consequence of the collision rate τ between molecules being large compared to the frequency of the sound wave ω ≫ 1/ τ. For second sound, the Umklapp rate τ u has to be small compared to the oscillation frequency ω ≪ 1/ τ u for energy and momentum conservation. However analogous to gasses, the relaxation time τ N describing the collisions has to be large with respect to the frequency ω ≫ 1/ τ N, leaving a window: