When the quasiparticle's mean free path ℓ ℓ ≪ ≪ λ λ (equivalently, relaxation time τ τ ≪ ≪ 1 / ω ω ), ordinary sound waves ("first sound") propagate with little absorption. But at low temperatures T (where τ τ and ℓ ℓ scale as T − − 2 ), the mean free path exceeds λ λ , and as a result the collision functional St [ f ] ≈ ≈ 0 . Zero sound occurs in this collisionless limit.