Space harmonic expansion has been used successfully to model sound transmission through infinite, periodically rib-stiffened double leaf partitions. Since the solution to this method is obtained in a series form, computational accuracy needs to be balanced with computational cost as calculation time increases with the number of space harmonic terms. The aim of this article is to provide a criterion to decrease the computational time when using space harmonic analysis. The new criterion helps to select the appropriate space harmonics to be included in the solution based on frequency and the angle of incidence of sound waves. The results are verified by comparing with experimental data available in the literature. For the partition investigated, the computational time is reduced by a factor of ten without compromising the accuracy of the result.