It follows that the particle velocity and the sound pressure along the direction of propagation of the sound wave x are given by v ( r , t ) = ∂ ∂ δ δ ∂ ∂ t ( r , t ) = ω ω δ δ m cos ⁡ ⁡ ( k ⋅ ⋅ r − − ω ω t + φ φ δ δ , 0 + π π 2 ) = v m cos ⁡ ⁡ ( k ⋅ ⋅ r − − ω ω t + φ φ v , 0 ) , p ( r , t ) = − − ρ ρ c 2 ∂ ∂ δ δ ∂ ∂ x ( r , t ) = ρ ρ c 2 k x δ δ m cos ⁡ ⁡ ( k ⋅ ⋅ r − − ω ω t + φ φ δ δ , 0 + π π 2 ) = p m cos ⁡ ⁡ ( k ⋅ ⋅ r − − ω ω t + φ φ p , 0 ) , where