Acoustic medium Sound travels through both the air and structure, and both paths must be considered when designing sound isolating walls and ceilings. To eliminate air borne sound all air paths between the areas must be eliminated. This is achieved by making seams airtight and closing all sound leaks. To eliminate structure-borne noise one must create isolation systems that reduce mechanical connections between those structures.
Mass Adding mass to a partition reduces the transmission of sound. This is often achieved by adding additional layers of gypsum. It is preferable to have non symmetrical leaves, for example with different thickness gypsum. The effect of adding multiple layers of gypsum wallboard to a frame also varies depending on the framing type and configuration. Doubling the mass of a partition does not double the STC, as the STC is calculated from a non-linear decibel sound transmission loss measurement. So, whereas installing an additional layer of gypsum wallboard to a light-gauge (25-ga. or lighter)
steel stud partition will result in about a 5 STC-point increase, doing the same on single wood or single heavy-gauge steel will result in only 2 to 3 additional STC points. 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 , and lightweight drywall has a nominal density of . 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 converting acoustical energy into other forms, 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. When two leaves are rigidly tied or coupled by a stud, the sound isolation of the system depends on the stiffness of the stud. Light-gauge (25-gauge or lighter) provides better sound isolation than 16-20-gauge steel, and noticeably better performance than wood studs. When heavy gauge steel or wood studs are spaced on center, additional resonances form which further lower the sound isolation performance of a partition. For typical gypsum stud walls, this resonance occurs in the 100–160 Hz region and is thought to be a hybrid of the mass-air-mass resonance and a bending mode resonance caused when a plate is closely supported by stiff members. Single metal stud partitions are more effective than single wood stud partitions, and have been shown to increase the STC rating by up to 10 points. However, there is little difference between metal and wood studs when used in double stud partitions. Damping generally increases the sound isolation of partitions, particularly at mid-and-high frequencies. Damping is also used to improve the sound isolation performance of
glazing assemblies. Laminated glazing, which consists of a
Polyvinyl butyral (or PVB) inter-layer, performs better acoustically than a non-laminated glass of equivalent thickness.
Sound leakage All holes and gaps should be filled and the enclosure hermetically sealed for sound isolation to be effective. The table below illustrates sound proofing test results from a wall partition that has a theoretical maximum loss of 40 dB from one room to the next and a partition area of 10 meters squared. Even small open gaps and holes in the partition have a disproportionate reduction in sound proofing. A 5% opening in the partition, which offers unrestricted sound transmission from one room to the next, caused the transmission loss to reduce from 40 dB to 13 dB. A 0.1% open area will reduce the transmission loss from 40 dB to 30 dB, which is typical of walls where
caulking has not been applied effectively Partitions that are inadequately sealed and contain back-to-back electrical boxes, untreated recessed lighting and unsealed pipes offer flanking paths for sound and significant leakage. Acoustic joint tapes and caulking have been used to improve sound isolation since the early 1930s. Although the applications of tapes was largely limited to defense and industrial applications such as naval vessels and aircraft in the past, recent research has proven the effectiveness of sealing gaps and thereby improving the sound isolation performance of a partition.
Flanking Building codes typically allow for a 5-point tolerance between the lab-tested and field-measured STC rating; however, studies have shown that even in well-built and sealed installations the difference between the lab and field rating is highly dependent on the type of assembly. == Special variations of STC ==