In a sound wave the velocities v _n and v _S are assumed to be small,† and the thermodynamic quantities almost equal to their equilibrium values. The propagation of sound in helium II is described by the system of hydrodynamic equations, Eqs. 8-22 to 8-25, which in this case may be linearized, yielding (10-1) ∂ ρ ∂ t +   div   j = 0 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429502897/8a91c217-57b7-40b4-af09-9dd4799d50ea/content/eq296.tif"/> (10-2) ∂ ρ σ ∂ t + ρ σ   div   v n = 0     ( σ ρ =   S ) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429502897/8a91c217-57b7-40b4-af09-9dd4799d50ea/content/eq297.tif"/> (10-3) ∂ j ∂ t + ∇ p   = 0 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429502897/8a91c217-57b7-40b4-af09-9dd4799d50ea/content/eq298.tif"/> (10-4) ∂ v s ∂ t + ∇ μ   = 0 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429502897/8a91c217-57b7-40b4-af09-9dd4799d50ea/content/eq299.tif"/>