Higher-order vibration modes of a loudspeaker diaphragm are represented as multipole sources of a sound field. As each normal mode corresponds to a multipole series, the radiation induced by a vibration pattern containing many normal modes is the image of a linear mapping of the participation factors to the coefficients of a multipole series. In terms of linear algebra, the participation factors and the set of corresponding coefficients are written as vectors. The generated sound field is then computed as a superposition from the multipole sources with strengths given by the coefficients in the series. Correction factors are introduced to count for the finite extent and other geometry related effects of the source. The higher- order normal modes form separate loads for the driving circuit. This method allows one to apply lumped parameter models at frequencies where the loudspeaker diaphragm does not move as a piston anymore.