− \frac{n^{2}π^{2}}{L_{x}^{2}} ψ − \frac{n^{2}π^{2}}{L_{y}^{2}} ψ − \frac{n^{2}π^{2}}{L_{z}^{2}} ψ = − \frac{1}{c^{2}} ω^{2} ψ

\frac{∂^{2}ψ}{∂x^{2}} + \frac{∂^{2}ψ}{∂y^{2}} + \frac{∂^{2}ψ}{∂z^{2}} + = \frac{1}{c^{2}} \frac{∂^{2}ψ}{∂t^{2}}

ψ(r, θ, ϕ, t)

\frac{∂^{2}ψ}{∂r^{2}} + \frac{2}{r} \frac{∂ψ}{∂r} = \frac{1}{v^{2}} \frac{∂^{2}ψ}{∂t^{2}}

ψ(r) = A sin(kr − ωt)

ψ(r) = A (\frac{1}{r}) sin(kr − ωt)

\frac{1}{r^{2}} \frac{∂}{∂r} (r^{2} \frac{∂ψ}{∂r} ) + \frac{1}{r^{2} sin^{2}(θ)} \frac{∂}{∂θ} (sin(θ)\frac{∂ψ}{∂θ} ) + \frac{1}{r^{2} sin^{2}(θ)} \frac{∂^{2}ψ}{∂ϕ^{2}} = \frac{1}{v^{2}} \frac{∂^{2}ψ}{∂t^{2}}