Covers a laboratory procedure for measuring airborne and impact sound insulation of an access floor with a plenum of specified height mounted below an acoustical barrier separating two rooms of a defined test facility. The procedure uses a laboratory space arranged so that it simulates a pair of horizontally adjacent rooms or offices sharing a common access floor system, plenum space and dividing wall. The wall extends from ceiling to upperside of floor system which at the junction is continuous or discontinuous. The procedure may be extended to the study of the additional sound insulation that can be attained by auxiliary systems, for instance material used as backing for all, or part of the floor, or as plenum barriers.