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STC 35 STC 43 STC 50 STC 60 to 64 2. Sound Transmission Classes of Various Framed Partitions

The ideal soundproof partition is airtight, heavy, and limp. A thick, hanging sheet of soft lead that is sealed around the edges fulfills all these requirements. It is expensive and unattractive, however, so we detail partitions of a combination of standard materials in such a way that we incorporate the necessary qualities of airtightness, heaviness, and limpness.

1. The degree of resistance of a partition to the transmission of sound is measured in units of decibels and is called its sound transmission class (STC). For a variety of partition construction details, STCs arc tabulated in references such as the Gypsum Association's Fire Resistance Manual and the National Concrete Masonry Association's Tek Notes. The accompanying table gives recommended minimum STCs for residential partitions in a variety of situations; higher values than these are desirable. Because background noise masks the intelligibility of sounds, partitions of lesser acoustical quality are considered acceptable in noisier neighborhoods.

2. This sampling of partition details illustrates the fundamentals of detailing a framed partition for acoustical privacy. The STCs of these partitions may be related to the preceding table. All of these constructions utilize readily available, inexpensive materials: gypsum wallboard and wood stud framing. (Similar details have been developed for steel stud framing.) Each partition is made airtight by using acoustical sealant around the edges, by avoiding electrical outlets that pierce the wall, and by using heavy, tightlv gasketed doors. The lowest-rated partition (far left) uses gypsum board attached directlv to the studs. In the next detail to the right, limpness is created by mounting the gypsum board on one side of the partition on resilient sheet metal channels that absorb vibrations. This results in an 8-decibel improvement in performance. A sound attenuation blanket of mineral fiber adds a further 7 decibels (second from right). The best partition (far right) is made heavy by applying additional layers of gypsum board and by making the two sides of the partition unequal in mass so that they will not vibrate at the same frequencies. The channels, sealants, and batts are all items that are available from stock.

3. Masonry partitions generally have fairly high STCs. Masonry is a heavy material. Some types of concrete masonry are fairly porous but can be made airtight with paint or plaster. Limpness is a difficult quality to achieve in masonry itself but can be added bv j j mounting resilient channels to a masonry wall and attaching a gypsum board finish layer to the channels.

4. An opening in or around a door—a keyhole or ventilation grill, an undercut bottom edge, or a crack between the door and frame—can make an otherwise effective partition almost transparent to sound. Doors in privacy partitions should be avoided where possible. If a door is necessary, it should be solid-core wood or composite steel designed and tested for acoustic isolation. These are typical acoustical gasket-ing details, using commercially available components, to seal the cracks around a door. (See also Weather stripped Crack, p. 40). >

Plaster one side

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