The sound insulation of building partitions at low frequencies depends on the particular configuration of rooms that are separated by a given partition, not only on the properties of the partition itself. When the modes of the rooms match (e.g., when rooms are identical), the sound insulation can be reduced substantially. If the partition responds in a resonant way, the reduction can also be due to the matching of modes of the partition with those of the rooms. In the current paper the phenomena involved are studied by means of the numerical simulation. Both the modal approach and the finite/boundary element method are used for the calculation of the difference of the averaged sound-pressure level in the 3-D case, yielding both the narrow-band results and band-averaged ones. The modal approach offers faster calculations, but is less flexible regarding rooms' geometry and partition model. The study of the changes of sound insulation due to slight changes to the original dimensions has been carried out for a great variety of combinations of original rooms' dimensions, and for several wall types (single leaf nonresonant, single leaf resonant, double leaf).