This paper presents first an acoustic mode theory for continuous surfaces. The necessary step of basis function normalization is made and is then introduced into the discrete theory to make the two theories consistent. Significant reductions in degrees-of-freedom are illustrated using an example of a cylinder with endcaps as the source surface. The analysis shows that the acoustic modes and radiation efficiencies vary with frequency and, notably, field geometry. A procedure is then described for developing acoustic modal filters to construct a cost function for active sound control.