There is no generally accepted standard methodology to determine the dynamic constants of whole cross laminated timber (CLT) plates. Combined with the observation that CLT walls or floors are installed in situ under very different conditions to laboratory measurement conditions a methodology is sought to determine the sound insulation of CLT plates installed under arbitrary conditions without accurate information concerning the dynamic constants (and boundary conditions). The methodology may have applications where a panel can be tested in situ e.g. in pre-completion testing. In the laboratory measurement a CLT plate was mounted on the laboratory floor on two wooden joists placed at arbitrary positions to support the weight of the plate to represent a possible installation in situ.