Question: Is it better to use lightweight or normal weight concrete blocks to control sound transmission through demising walls between hotel rooms?
Answer: Lighter blocks reduce the weight and increase the thermal insulating properties, but they are not the best choice for lowering sound transmission. Reduction of sound transmission is related to the density of the wall: the higher the density of the units the better the sound transmission reduction. For this reason normal weight units are better than lightweight blocks for reducing sound transmission.

The best way to determine the STC rating of an assembly is to perform laboratory tests and review the results. ASTM E 90-00, Standard Test Method for Laboratory Measurement of Airborne-Sound Transmission Loss of Building Partitions, is used to determine the STC rating of building partitions.

If test data is not available, Document 0302, "Standard Method for Determining the Sound Transmission Class Rating for Masonry Walls", published by The Masonry Society, can be used. Data in this document is on the conservative side and typically underestimates the actual STC rating.

The rating is determined by the equation STC = 0.18W + 40 where W is equal to the weight of the wall expressed in terms of pounds per square foot. The equation is used only for uncoated fine or medium textured concrete masonry units. Coarse texture concrete masonry units may allow airborne sound to enter the wall system. Surface treatments may be needed to increase the STC rating when coarse textured units are used.

The equation cannot be used if the wall system is thinner than 3 inches. The units must be laid in full mortar joints, and all holes, voids, or cracks in the masonry must be solidly filled with mortar. Table 8.3-2 in the above mentioned document lists the STC ratings for different masonry walls.

It is important to recognize that very small openings in a masonry wall significantly affect sound transmission through it. Openings through the wall for a pipe or conduit greatly increase sound transmission if the penetration is not properly detailed. Back-to-back receptacle boxes in walls or receptacle boxes not coated on the backside to reduce sound transmission greatly increase the sound transmission through the masonry walls.

Any joints in the wall and the joint along the top of the wall significantly impact the sound transmission if not properly detailed. These joints must be covered with elastomeric sealant on the exterior face. Space between the top of the wall and the underside of the structure above must be filled with mortar or grout if it is not designed as a movement joint, or filled with foam or fiber material where movement must be accommodated.

Doors in demising walls are also a common cause of sound transmission problems. Using doors with high sound transmission ratings, as well as proper detailing, are essential to prevent sound transmission through demising walls in hotels.