The acoustical separation of façades can be described by using either sound transmission class (STC) or outside-inside transmission class (OITC). The measurement process for STC is defined in ASTM E90, Standard Method for Laboratory Measurement of Airborne Sound Transmission. This procedure uses two diffuse fields created by reverberation chambers to measure the amount of sound energy blocked by a tested assembly. The resultant transmission loss is plotted and ASTM E413, Classification for Rating Sound Insulation, is used to generate the STC value. It is important to understand STC was originally developed to evaluate speech attenuation. Consequently, it may not show the whole picture for exterior noise sources that are generated at frequencies lower than speech (i.e. below 125 Hz). Busses, heavy rail, and rooftop restaurants will need a full frequency evaluation to determine the appropriate façade construction.
OITC was developed in 1990 and is defined by ASTM E1332, Standard Classification for Rating Outdoor-indoor Sound Attenuation. This measurement was intended to evaluate attenuation due to typical exterior noises such as busses, construction, and sirens, rather than speech frequencies. The OITC rating emphasizes low-frequency noise in the evaluation more than the STC rating since the ambient environment has more of these sound sources compared to speech. Figure 1 (page 77) presents typical STC and OITC rating of severa`l assemblies.
The single greatest factor influencing sound isolation is mass. Entries A and B in Figure 1 highlight an underlying rule of sound isolation called the Mass Law, wherein a doubling of mass (or a doubling of frequency) produces a 6 dB increase in sound isolation.
A dual-mass system can often help improve acoustical separation, but the details are important and not always intuitive. In configurations such as entries A and B in Figure 1, two moving masses (gypsum wall board [GWB]) are arranged with a significant airspace separating them. The air trapped in the cavity by the drywall (i.e. the mass-air-mass system) resonates at about 50 to 70 Hz for an insulated stud wall, which is below the frequency included in the STC or OITC evaluation. Due to this, the decreased transmission loss associated with the mass-air-mass resonance does not reduce the single-number rating. Generally, airspaces less than 76 mm (3 in.) should be avoided in framed assemblies. An increase in airspace (e.g. up to 203 mm [8 in.]) can help with the reduction of low-frequency sounds as well as the overall transmission loss performance.
Unlike framed partitions, typical glazing systems have significantly smaller airspaces (e.g. 13 mm [½ in.]), and this results in resonances in the range of 150 to 190 Hz, which is more noticeable to people, relevant to speech privacy, and included in the STC and OITC evaluation. As an example, 13-mm monolithic glass has a rating of STC 34 (entry D in Figure 1), while a 25-mm insulated glass unit (IGU) consisting of 6 mm (¼ in.) glass, 13 mm airspace, and 6 mm glass (entry G in Figure 1) will result in STC 33 for the same amount of glass. It is also important to note the effect of a large airspace between glazing lites by comparing entries G and H in Figure 1.
Laminated glass can improve acoustical separation by allowing for an impedance change in the material. When sounds or vibrations experience changes in density, part of the noise is reflected back in the direction it originated. Thus, by including a thin layer of polyvinyl butyral (PVB) or ethylene-vinyl acetate between two thin layers of glass, the change in density between the glass and the lamination layer causes the reflection of some of the sound energy. A piece of 6-mm plate glass (entry C in Figure 1) provides acoustical separation of STC 31, while a piece of 6-mm laminated glass (entry E in Figure 1) offers acoustical separation of STC 35.
The interaction of the façade with the inside of the building and the provision of an appropriate level of acoustical separation requires specific attention to detail. Partitions requiring acoustical separation higher than STC 35 should be lined up with a mullion, as it is difficult—nearly impossible—to provide an appropriate seal at the glazing itself. In the past, the only true method of giving the right level of acoustical separation was to provide blocks on the mullion to match the thickness of the stud, and to extend the drywall past the stud row into the mullion (Figure 2).