The influence of plenum absorption on flanking transmission through suspended ceilings

In non-residential buildings, rooms are often separated by lightweight partition walls beneath a suspended ceiling. The air volume between this suspended ceiling and the solid ceiling above it, called a “plenum”, forms a bypass for sound propagation. The absorption at three walls of this plenum according to ISO 140-9 is supposed to give a good approximation of the “best real case”, where the rooms under consideration are situated far away from any reflecting solid walls of the building, e.g. facades. In cases where reflecting walls flank or even surround the lightweight construction under consideration, lower results are expected. In those cases, the standard measurements yield optimistic results. Measurements performed with a model 1:10 in size are used to discuss different setups of absorption in the plenum with respect to their ability to represent different real situations.