  This paper presents a study of the forced acoustical response of an open
cavity from the perspective of modal expansion. Based on the coupled mode
theory, it is shown that the sound pressure distribution of an open cavity
excited by a point source placed within the cavity can be expanded by a set of
frequency-dependent eigenmodes, which are derived from the coupling between the
cavity and a semi-infinite space. The calculation of the acoustical responses
for baffled and unbaffled open cavities indicates that the proposed modal
expansion converges with only a few frequency-dependent eigenmodes in the
frequency range of interest. The results of this study eliminate the ambiguity
involving the selection of appropriate basis functions, in modal expansion for
the forced response problem in open cavities.
