(in the mid STC 40s to the mid STC 50s) than equally thick framed walls. However the additional weight, added complexity of construction, and poor thermal insulation tend to limit masonry wall partitions as a viable sound isolation solution in many building construction projects. In recent years, gypsum board manufacturers have started to offer lightweight drywall board: Normal weight gypsum has a nominal density of 43pcf and lightweight drywall has a nominal density of 36pcf. This does not have a large effect on the STC rating, though lightweight gypsum can significantly degrade the low frequency performance of a partition as compared to normal weight gypsum. Sound absorption Sound absorption entails turning acoustical energy into some other form of energy, usually heat. Adding absorptive materials to the interior surfaces of rooms, for example fabric-faced fiberglass panels and thick curtains, will result in a decrease of reverberated sound energy within the room. However, absorptive interior surface treatments of this kind do not significantly improve the sound transmission class. Installing absorptive insulation, for example fiberglass batts and blow-in cellulose, into the wall or ceiling cavities does increase the sound transmission class significantly. The presence of insulation in single 2x4 wood stud framing spaced 16" (406 mm) on-center results in only a few STC points. This is because a wall with 2x4 wood stud framing spaced 16" develops significant resonances which are not mitigated by the cavity insulation. In contrast, adding standard fiberglass insulation to an otherwise empty cavity in light-gauge (25-gauge or lighter) steel stud partitions can result in a nearly 10 STC-point improvement. Other studies have shown that fibrous insulation materials, such as mineral wool, can increase the STC by 5 to 8 points. Stiffness The effect of stiffness on sound isolation can relate to either the material stiffness of the sound isolating material or the stiffness caused by framing methods. Framing methods Structurally decoupling the gypsum wallboard panels from the partition framing can result in a large increase in sound isolation when installed correctly. Examples of structural decoupling in building construction include resilient channels, sound isolation clips and hat channels, and staggered- or double-stud framing. The STC results of decoupling in wall and ceiling assemblies varies significantly depending on the framing type, air cavity volume, and decoupling material type. Great care must be taken in each type of decoupled partition construction, as any fastener that becomes mechanically (rigidly) coupled to the