Depending on the space in question, the attention given to acoustical sound attenuation is as important as the location of power and communications outlets, the performance of lighting systems, and the efficiency of energy usage. This is especially true for interior spaces such as a teleconferencing rooms, clinics, theaters, or open-plan offices. As such, it's important that you understand the basic principles of architectural acoustics: the technology of designing spaces, structures, and mechanical/electrical systems to meet the needs of the occupants.

Let's look at an excellent example of how sound attenuating electrical designs and installation methods work together to get the job done.

Building design case history

The 2062-seat concert hall in the Morton H. Meyerson Symphony Center in Dallas, Tex. is an excellent example of how vibratory and aerodynamic noise can be reduced. The building, made primarily of reinforced concrete, is located in the flight path of the busy Dallas-Ft.Worth airport and near two major streets. The designated noise-critical spaces are the audience chamber, performance platform, sound locks, inner lobbies, assembly areas, control rooms, rehearsal rooms, warm-up rooms, and lighting positions.

To make sure that no mechanical noise is carried into the concert hall from vibrations generated within the facility, the entire utilities plant budding is separated from the main building by 2-in. joints.

Located in the utilities building, the electric service room contains five step-down transformers and distribution boards, all of which are installed on a floating slab. The slab, designed for total weight support, was cast in place and then jacked-up to provide air gap separation supported by double-deflection molded neoprene isolators, as shown in Fig. 1. Conduits penetrating the jacked-up slab or walls use a sleeve-within-a-sleeve construction. The sleeve around each conduit is packed with polyethylene foam and sealed with non-hardening mastic compound.

In addition, transformers on the jacked-up slab, shown in Fig. 2 on page 108, are mounted on 5/16-in. thick neoprene pads ribbed or waffled on both sides, providing static deflection of not less than 0.03 in.

Penetrations of walls and slabs for conduits leaving the utility building have sleeved openings that are packed and caulked airtight, as shown in Fig. 3 (see page 112). Where a conduit or cable passes through a wall, a steel sleeve is grouted into the structure; where the conduit or cable passes through a floor or ceiling slab, the sleeve is cast in place. The internal dia of a sleeve is 2 in. larger than the outside dia of the conduit passing through it. After the piping is installed, the electrical contractor checks the clearance and corrects it, when necessary, to within 1/2 in. Then, the void is packed full-depth with glass fiber and sealed at both ends, 1-in. deep, with a nonaging, nonhardening mastic compound backed by a polyethylene foam rod that is placed, like a bead, around the opening.

Motors and electrical equipment are supported on slab by springs and neoprene mounts or, where hung from slab, on springs and neoprene hangers. Electrical conductors are carried in a minimum 36-in. long flexible conduit having a 360[degrees]loop. A separate grounding conductor is run within the flexible conduit and terminated at each end at a grounding bushing.

Located outside the noise-critical spaces, the dimmer racks are installed 3 ft clear of adjacent walls and on metal and waffle neoprene isolation pads. All wiring connections to the dimmer rack enter through neoprene gasketed collars with conduit grounding bushings, as shown in Fig. 4 (see page 113).

Lighting fixture and lamp type selection was reviewed by the lighting consultant (Fisher & Marantz, New York City) and the acoustical consultant to make sure that the incandescent lamps used in the concert hall and other noise-critical areas would not "buzz" when dimmed. The selection was limited to "A" lamps, "R" reflector lamps, and tungsten-halogen, endprong, reflector lamps. No ballasted fixtures are located within the noise-critical spaces.

To minimize aerodynamic turbulence (see sidebar on page 114) at the supply outlets, the acoustical consultant asked that the maximum supply air velocity in the main ducts be held to 1500 cubic feet per min (cfm), and in the concert hall to 600 cfm. These requirement were met by having the main ducts feed into a concrete chamber lined with 2-in. fiberglass. This configuration slows the velocity of air exiting the chamber to 600 cfm or less. Air leaving the chamber travels through nozzles that open onto the concert hall. In addition, double-level concrete floors are used to create return air plenums with acoustic linings.

The acoustical consultant was Artec Consultants, Inc., and the mechanical-electrical engineering design was by Edwards & Zuck, with Anthony M. Cottone as project executive and Harry Hagenfeld as project electrical engineer. Both firms are in New York City.

The installing electrical contractor was J.D. Keys Electric Co., Dallas, Tex., with Carl Tyner, as project coordinator.