If u is a superposition of such waves with weighting function φ, then u ( t , x , y , z ) = 1 4 π π c ∭ ∭ φ φ ( ξ ξ , η η , ζ ζ ) δ δ ( r − − c t ) r d ξ ξ d η η d ζ ζ ; the denominator 4 πc is a convenience.