The eigenvalue λ introduced in this equation is related to the eigenfrequency f and the angular frequency ω, through λ = −i2πf = −iω. Because they are independent of the pressure, the solver ignores any dipole and monopole sources unless a coupled eigenvalue problem is being solved.

Equation 11-6 applies to the 3D case. The equations solved in eigenfrequency studies in lower dimensions and for axisymmetric geometries are obtained from their time-harmonic counterparts, given in the previous subsection, by the substitution ω2 → −λ2.