Checklist for Mechanical System Noise Control

Mechanical Rooms and Equipment

1. Locate mechanical equipment rooms far away from sensitive rooms, use non- sensitive buffer zones between, and size equipment rooms to have sufficient space for operation, inspection, and maintenance of all equipment. When sufficient floor space and vertical clearance are provided, mechanical engineers can design air ducts and piping to have smooth turns and transitions so turbulence will be less likely to occur. Fans, chillers, and the like should not be located close to walls where sound energy can build up in the narrow space between. Noise transmission problems also can be prevented if vibration isolators, equipment bases, and all structural penetrations are accessible for routine inspection.

2. Specify the quietest equipment available based on octave-band data or size equipment to operate at low noise levels (e.g., fan coil units with low- medium-high motor speeds can be sized to operate on medium speed at the maximum thermal demand conditions).

3. Use heavyweight wall and floor-ceiling constructions with high transmission loss TL performance to enclose mechanical rooms. The mechanical rooms in large buildings can be a major source of noise, sometimes exceeding 100 dBA. In critical situations, where mechanical rooms can't be located on grade and away from sensitive spaces, resilient isolators, floated floors, high IL-rated doors, and resiliently suspended ceilings underneath the structural floor all may be required.

4. Treat the walls and ceiling of mechanical rooms with generous amounts of thick, sound-absorbing materials to reduce the buildup of airborne sound.

5. Use soft, resilient materials (springs or pads) under the wearing surface of floors to isolate mechanical rooms from the building structure (called floated floors) or between the bases or supports of vibrating equipment and the structure (called resilient isolators) to minimize the transfer of vibrations into the structure.

6. Roof-mounted mechanical equipment subject to wind loading (e.g., cooling towers, packaged air-conditioning units) and all mechanical equipment subject to earthquake forces may require special vibration-isolation mounts. These well-anchored lateral and vertical restraints must be carefully selected and in stalled to avoid short-circuiting the vibration-isolation system.

Vibration Isolation

1. Prefabricated, resilient equipment mounts and bases are commercially available and , when properly selected and installed, can provide stable support to vibrating equipment. Use isolation mounts such as unhoused steel springs, ribbed or waffle-shaped neoprene, or precompressed glass-fiber pads, de pending on the vibration to be isolated and the support conditions.

2. Locate vibrating equipment near columns or above load-bearing walls to obtain better structural support. Long floor spans (> 20 ft) have lower natural frequencies than on-grade slabs and , therefore, require isolators with larger static deflections.

3. Use isolation hangers to isolate all pipe connections for a considerable distance from vibrating equipment (> 150 times pipe diameter).

4. Use flexible (e.g., resilient braided section) and “floppy” (e.g., full 360’ loop) electrical conduit connections to vibrating equipment.

5. Pipes carrying water at high velocities should be isolated from the building structure and wrapped with dense lagging materials (e.g., lead sheet, vinyl- covered glass fiber) to reduce the transmission of water flow noise. Use rein forced flexible pipe connections to break the vibration path along the walls of piping.

Air-Distribution Systems

1. Use low-velocity and low-pressure air-distribution systems to control airflow noise generation and conserve energy. Avoid complicated duct layouts (e.g., crisscrossing of ducts, multiple dogleg turns) and abrupt changes in duct cross-sectional area.

2. Avoid constructions such as untreated mechanical ducts, chases and shafts, or rigid conduits that can act as “speaking tubes” to transmit sound from one area to another. To avoid cross talk through ducts, install widely separated air outlets, locate air inlets to plenums away from common walls, and line common ducts with glass fiber (at least 1 in thick), or use prefabricated, sound-attenuating mufflers. Where ducts pass through walls and floor-ceiling constructions, resiliently isolate them from the structure.

3. To effectively seal openings around mechanical penetrations through enclosing constructions of mechanical rooms (and through other critical barriers), use low-density fibrous packing to completely fill 1/2-in-perimeter ring of open space and use non-hardening caulking on both sides of opening to achieve an airtight seal.

4. To prevent low-frequency rumble from air turbulence in mechanical ducts, be sure to provide smooth transitions and gradual turns and takeoffs. In variable air volume systems, locate terminal control devices far upstream ( > 15 ft) from diffusers and registers so generated airflow noise can be attenuated by lined ducts before reaching occupied rooms.

5. Use circular ducts, or flat oval and rectangular ducts with low aspect ratios (ratio of side dimensions < 3:1), which are less susceptible to vibration due to their greater stiffness.

6. Use flexible duct connections ( > 3-in separation) to break the vibration path along walls of sheet metal ducts. As in most vibration isolation, the goal is to interrupt the transmission path by preventing metal-to-metal contact.

7. Where ducts pass through noisy spaces before reaching quiet spaces, lagging may be required to prevent break-in of unwanted sound energy. Use circular ducts or sound-isolating plenums to compensate for the poor low-frequency noise isolation of rectangular duct walls.

Next: Vibration and Acoustical Testing/Engineering Services

Prev: Mechanical Equipment Noise Level Data

top of page   Home