Compound Barriers

Most partitions are built of light, upright framing mem- i( bers with plaster or gypsum wallboard surfaces attached to both sides. This construction does not provide a very good sound barrier. Adding layers of gypsum wallboard to one or both sides increases the wall's mass and improves acoustic performance.

Gypsum wallboard consists of fire-rated sheets of gypsum, which has been heated to a high temperature, plus additives. These are then sandwiched between sheets of special paper. Gypsum wallboard is not very heavy or thick but provides fair sound attenuation. The best construction detail for blocking sound uses multiple layers of gypsum wallboard with a resilient separation between the two faces of the partition, and with absorptive material in the stud space. The wallboard joints must be perfectly sealed. Gypsum wallboard will resonate unless it is attached directly to a solid substrate without an air space, so that it will absorb low-frequency sounds. It is highly reflective of higher frequencies.

Compound barriers or cavity walls improve transmission loss when the void between the two sides of the wall is filled with porous, sound-absorbent material. This decreases the stiffness of the compound structure, and absorbs sound energy reflecting back and forth between the inside wall surfaces. Steel channel studs are used to frame partitions and are covered with gypsum wallboard. Light gauge steel studs are lightly resilient, which helps the wall attenuate sound. Heavy gauge steel studs and wood studs are stiffer and offer less sound attenuation. Steel or wood studs do not add significantly to the wall's sound absorption.

When one layer of gypsum wallboard is attached to the framing with resilient metal clips instead of tight screws, structure-borne transmission of sound through the partition is reduced substantially. Resilient clips and channels (Fig. 53-4) are usually made of light-gauge sheet metal, and are used between studs or joists and the finished gypsum wallboard or plaster surface. They are highly effective with wood joists and studs. By breaking the rigid connection between the two faces of the partition, resilient channels and clips permit room surfaces to vibrate normally without transmitting vibrating motions and the associated noise to the supporting structure. They reduce the sound transmission through the partition or ceiling.

Gypsum wallboard

-Wood stud

Acoustic fiberglass batt insulation

Acoustic fiberglass batt insulation

Staggered studs

Figure 53-5 Staggered-stud partition.

Where the studs are used in two unconnected rows, their stiffness isn't an issue. Staggered-stud partitions (Fig. 53-5) for reducing sound transmission between rooms are framed with two separate rows of studs arranged in a zigzag fashion and supporting opposite faces of the partition. This type of wall is often used in recording studios. A fiberglass blanket is often inserted between the rows of studs. A stud wall with staggered studs is better than a single-material or common stud wall.

Gypsum wallboard

Wood Stud

Acoustic insulation

Resilient channel attaches wallboard to studs.

Figure 53-4 Resilient channels.

Resilient channel attaches wallboard to studs.

Acoustic insulation

Figure 53-4 Resilient channels.

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