The wave equation can be solved using the technique of separation of variables. To obtain a solution with constant frequencies, let us first Fourier-transform the wave equation in time as Ψ Ψ ( r , t ) = ∫ ∫ − − ∞ ∞ ∞ ∞ Ψ Ψ ( r , ω ω ) e − − i ω ω t d ω ω ,