January 15, 2014 | Written by: Nicole Cuff | Original Publication: FacilitiesNet (BOMA)

Good Noise Control Principles for HVAC Design

Noise control considerations and low background sound levels are critical for many kinds of buildings: not only listening spaces like performance halls, but also residences, offices, schools, and laboratories. Planning for good acoustical design is best when started early in a project. When the acoustics for HVAC systems are included early in the design, noise control is not a burden and can be seamlessly integrated. Sometimes acoustical concerns are considered at a late stage in the design or after the project is built, when it is much more difficult to incorporate appropriate noise control. Below, we describe some basic noise control design principles for HVAC systems.

HVAC location is key
One of the most important principles for noise control in HVAC design is to locate mechanical sources away from noise-sensitive rooms. For the most sensitive projects like performance halls, noisy mechanical equipment needs to be as far away from the noise-sensitive spaces as possible. For performance halls, this may mean that the mechanical equipment is in a structurally separate building from the performance room. For other important but less critical listening spaces, mechanical equipment should be in a basement room, below grade, again far away from any critical listening spaces.

It is good to locate “buffer” spaces next to mechanical rooms that are not sensitive to noise. These may include storage rooms, bathrooms, electrical closets, other mechanical rooms, and stairwells.

If mechanical rooms or mechanical units must be located adjacent to noise-sensitive spaces (perhaps if acoustics is considered at a late stage in the project), cost and complexity increase. In these cases, equipment needs to be enclosed in a massive, noise-blocking enclosure, and the very quietest equipment needs to be selected, which may be a cost premium to the project. The same principle applies to mechanical rooms: the walls may need to be thicker than originally planned and may require double stud wall partitions or double-wythe concrete masonry unit (CMU) walls.

Mechanical noise can transmit from the floor of one level to the deck of the level below. Noise can also transmit from sidewall to sidewall. This is important to remember when considering locating rooms near noise generating mechanical equipment. Even if the room with noise generating equipment is on a different floor level than a critical listening space, the noise can still propagate far and wide if noise transmission mitigation is not considered.

Fan coil units, variable or constant air volume boxes, cabinet unit heaters, heat pumps, and other mechanical units with rotating equipment should not be located above the ceiling of a critical listening space, furred in a wall, or (worst case) exposed in the room. Locate these types of equipment above the ceiling of an adjacent less critical space: a corridor or bathroom, or in the mechanical room if possible.

Isolating mechanical equipment vibration
Isolating equipment vibration from the building is also critical. Mechanical equipment produces vibration that, if not properly addressed, can transmit vibration (and vibration-induced noise) throughout the building. If the vibration-producing mechanical equipment is located far enough from noise-sensitive spaces, there is more of a safety factor to reduce vibration transmitted to the space (e.g. if the vibration isolation fails or is installed incorrectly). Any mechanical rotating equipment should be isolated in some manner to reduce the transfer of vibration, even if this equipment is located slab on grade. Consider, for example, an end-suction pump placed on slab on grade that is not vibration isolated. This pump transmits vibration into the slab. The slab, while large, will still vibrate; the vibration transmits throughout the slab and then into connecting walls, and then on to the level above. This transmitted vibration gets into walls, floors, or ceilings that will radiate airborne noise, which can be loud, distracting, or intrusive and increases the background sound levels for occupants.

Isolators that are used for vibration isolation come in many different varieties. Some isolators are neoprene type pads under floor-mounted equipment, which evenly support the equipment and absorb vibration. Others are spring isolators, which are point loaded to support equipment and absorb vibration. Engineers select the isolators according to the minimum operating speeds and weight of the equipment. Generally, equipment that runs more slowly – say, cooling towers – require larger, softer spring isolators. Isolators can also be specified to meet seismic requirements

and vibration isolation within one isolator. Piping and ductwork connected to the vibrating equipment can transmit noise as well, and must be considered. It is typical to use spring hangers to isolate piping and flexible ductwork connections for ducts connected to vibrating equipment.

Choosing efficient equipment
Another way to reduce noise and vibration transmission is to ensure that the mechanical equipment is selected for the most efficient operation as possible and that the systems considered are generally efficient. This may mean considering equipment that is not necessarily the lowest first-cost, but may have a low lifecycle cost. Additionally, a mechanically efficient design is a sustainable and LEED-friendly design.

It is also possible to reduce noise and vibration transmission by avoiding packaged rooftop units. Packaged units produce more significant levels of vibration and higher levels of noise than a standard air handling unit because of the compressor and condenser section. These units are also most often placed on the roof, directly above noise-sensitive spaces. For example, a packaged unit installed above an executive’s office on the top floor of an office building would require many acoustical upgrades to isolate sound and vibration. A smarter choice would be a traditional air-handling unit located in a mechanical room far from the executive’s office.

Other equipment alternatives can be good for noise control design. Alternate technologies to consider may include chilled beams or displacement ventilation, where less airflow is required to cool the space than traditional methods. For large equipment, this may include considering renewable energy sources to reduce the load on a chiller or heating plant – say, solar thermal panels. Otherwise, be sure to select fans in the air handling unit at an efficient operating point. For specifying engineers, this may mean choosing a larger fan than may be initially recommended by the fan manufacturer’s selection program.

Choosing efficient supply and return fans reduces the overall discharge noise from the fan and consequently the noise that is transmitted into the space. If the fan discharge noise contributes to a louder sound pressure level in the listening space than desired, silencers should be considered. Plan to keep pressure drop through the silencers low; if pressure drop is high, the silencers contribute their own source of noise to the space.

Another important source of noise to consider is the noise that is generated by the velocity of air flowing through ductwork. Fast-moving, turbulent air generates noise in addition to the noise created by mechanical equipment. Velocities must be kept in check – acoustical consultants can advise mechanical engineers on the specifics. Velocity restrictions can be relaxed somewhat if suitable sound-absorbing internal ductwork liner is used.

As noted above, best practice noise control design is much more difficult to adopt later on in the project or in an existing building. In existing buildings with HVAC equipment, the acoustical solution is often limited to architectural upgrades if the mechanical unit cannot be replaced or if the noise-generating equipment cannot be moved. These solutions are generally less effective than solutions that are possible earlier in the design of a new project.

When good noise control principles are considered early in the project design, the overall project acoustics are improved and can be implemented with less cost and greater ease.

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