Gives a laboratory method of measuring the airborne sound insulation of a suspended ceiling with a plenum of defined height mounted above an acoustical barrier which separates two rooms of a specified test facility and simulates a pair of horizontally adjacent, typical rooms sharing a common suspended ceiling system, plenum space and a dividing wall. May be extended to include study of composite ceiling systems with other components such as luminaires and ventilating systems and auxiliary systems, such as material used either as plenum barriers or as backing for the ceiling. Includes example of a test procedure, illustrates cross section and plan of the test facility. ALSO NUMBERED AS BS 2750: PART 9: 1987