Date: 2023-10-31 10:30:00
The machines that heat, cool, and ventilate our buildings are wonders of modern invention. In maintaining the comfort and health of our indoor environments, however, they generate noise that, if not attenuated properly, can adversely affect the comfort and health of the surrounding community.
Note: This article appears in the 2023 edition of AMCA inmotion magazine.
By JOHN SOFRA
A product of “large-scale developments such as growing mechanization, mobility, and particularly urbanization,”1 environmental noise, or noise pollution—defined as “unwanted or harmful outdoor sound created by human activity”2—is a growing concern around the globe. According to the World Health Organization (WHO), noise is among the top environmental threats to physical and mental health and well-being,3 causing hearing problems, sleep disorders, cardiovascular diseases, and annoyance.4
Among the primary sources of environmental noise—alongside road, rail, and air traffic; construction; and industrial activity—are buildings,5 namely the mechanical equipment used to control the temperature, humidity, and purity of the air within. Whether to comply with local ordinances or simply be a “good neighbor,” building owners and operators have several options for keeping mechanical-system noise in check.
This article will discuss means of curbing noise from the operation of ventilation fans, air-cooled chillers, and induced-draft cooling towers, specifically ventilation silencers (also known as acoustic duct silencers), fixed-blade acoustic louvers, double-walled barriers and enclosure panels, noise-control pergolas/stacks, and sound absorbers.
The first step in addressing any outdoor-noise problem is to understand the source of the noise, the receiver of the noise, and the path from the source to the receiver. The environmental surroundings also are important, as noise can propagate along many different paths. The factors most commonly affecting noise propagation outdoors are distance, atmospheric conditions, wind direction, terrain, and the presence or lack of wooded areas and buildings nearby.
The key to controlling noise produced by mechanical equipment is knowing how the equipment operates. A solution that mitigates noise but compromises equipment performance is a failed one.
In the design of a noise-control solution, equipment accessibility is important. If routine maintenance is difficult to perform, necessary work may not be completed, and equipment performance will suffer. What’s more, the life of the equipment will be shortened. It is important to enable portions of a system to be removed in the event of catastrophic equipment failure.
In summary, design of a successful noise-abatement solution takes into consideration the equipment manufacturer’s warranty, local electrical codes, equipment ventilation, local noise ordinances, equipment location with respect to critical areas, equipment dimensions, maintenance access, and structural issues (e.g., wind loading, seismic restraints, concrete footings, tie-ins, drainage).
Noise-Generating Mechanical Equipment
Mechanical equipment commonly benefitting from noise-control solutions includes ventilation fans, air-cooled chillers, and induced-draft cooling towers.
Ventilation fans. Typically, in the case of an exhaust fan mounted atop a commercial or industrial building, the fan intake is ducted with the fan discharging to atmosphere. Environmental noise is a function of the noise propagating from the fan discharge and radiating from the fan casing and motor drive train. Knowing how fans work and selecting the correct noise-control products can help keep noise within acceptable levels. Noisier fans tend to be oversized and operated under the design speed or undersized and operated above the design speed. Sometimes, re-adjusting a fan to run closer to its design speed and pressure will help to resolve a noise issue, lessening reliance on noise-control products and saving money.
Often, a fan-discharge silencer (Photo A) is all that is needed to solve a noise problem. Sometimes, however, a combination of fan silencer and acoustic barrier wall or enclosure (Photo B) is needed. With an acoustic barrier wall, accessibility and clearance between the wall and fan unit are important to meeting local electrical codes. If a full enclosure is applied, it is important to determine the amount of ventilation required to keep interior-temperature rise above ambient-air temperature within a set limit (e.g., 20˚F [10˚C]). Once the required ventilation is determined, enclosure ventilation silencers can be selected. These devices allow required ventilation airflow at minimum pressure loss while controlling noise propagated through enclosure ventilation openings. Sometimes, forced ventilation is needed, and, sometimes, convection (stack effect) ventilation will work. Taking all of these factors into consideration will keep a fan unit from overheating and shutting down.
Air-cooled chillers. The noise produced by an air-cooled chiller is a combination of mid-to-high-frequency sound generated by the screw compressor (bottom of the unit) and low-frequency sound generated by the upblast fan (top of the unit). It is important to understand an air-cooled chiller is a noise-generating system, and all components that generate noise must be considered. In many cases, the first attempt to reduce noise involves wrapping a compressor with a composite quilt and mass-loaded vinyl jacket. Although this will control some mid-to-high-frequency noise, rarely does it solve a system-noise problem, as it considers only one aspect of the system noise.
PHOTO A. Fan intake and discharge silencers. Photo courtesy of Kinetics Noise Control Inc.
PHOTO B. Dust-collector fan with acoustic barrier wall and silencer. Photo courtesy of Kinetics Noise Control Inc.
Silencer baffles, barrier walls, and fixed-blade acoustic louvers can be used to control air-cooled-chiller noise; the combination depends on the application, equipment footprint, location of noise-sensitive areas, and any local noise ordinance:
- If a barrier wall of sufficient height can be placed at the distance specified by the chiller manufacturer, one double-leaf acoustic access door is all that will be needed to achieve up to a 23-dBA noise reduction perpendicular to the outside of the wall.
- If the distance between a barrier wall and a chiller is less than recommended by the chiller manufacturer but within the limits of the local electrical code, a double-walled barrier with a skirt of fixed-blade acoustic louvers along the bottom of each wall parallel to the long dimension of the chiller (Photo C) will work. This design provides a supply of fresh ambient air along the bottom of the barrier-wall system, avoids any recirculation of heated air, and is focused on low pressure loss and required attenuation.
- If the distance between a barrier wall and a chiller is less than recommended by the chiller manufacturer but within the limits of the local electrical code and there is a need to control the noise propagating from the top of the barrier-wall system, an enclosure with acoustical louvers and roof baffles can be used to achieve a noise reduction of up to 10 dBA above the chiller.
Regardless of the combination used, it is important to achieve maximum noise control with minimal pressure loss.
Induced-draft cooling towers. The most common type of cooling tower consists of a large propeller fan discharging air upward, with air drawn into intakes on the sides and passed through a stream of water, some of which evaporates into the air stream, causing cooling. Induced-draft-cooling-tower noise is a combination of low-frequency fan noise; blade-passage tone (below 63 Hz); noise from the spraying and splashing of water; noise from the motor, belt, or gearbox; noise associated with fan speed (noise increases with fan speed); and discharge noise (typically 5 dB greater than intake noise).
Noise-control options consist of change of location and orientation, cooling-tower oversizing (expensive), low-noise fans, motor control (day-night operation, etc.), double-walled barriers, and intake and discharge silencers (Photo D).
Where cooling-tower noise control is concerned, materials of construction are important; budget, operating environment, and product lifespan all should be considered. The discharge air is 100-percent saturated with water vapor. Although stainless-steel discharge silencers work best, an owner looking to save costs may elect to go with G90 galvanized steel for an entire project, with the structural-steel support frame being factory-hot-dip-galvanized. In such cases, the owner will need to implement a strict maintenance program to prevent corrosion.
Ventilation silencers, fixed-blade acoustic louvers, double-walled barriers and enclosure panels, noise-control pergolas/stacks, and sound absorbers are all viable means of solving environmental-noise issues.
Ventilation silencers. Ventilation silencers are used to attenuate fan and cooling equipment and intake/discharge building openings, allowing fresh air exchange while controlling noise. Silencers come in many shapes (rectangular, circular, straight, elbow) and sizes and attenuate noise through means of sound reflection and absorption. Different types of silencers attenuate different sound frequencies. Take, for example, two silencers of a specific cross-sectional area and percent-open area. The one with fewer and thicker baffles is better at attenuating low- to mid-frequency noise, while the one with more and thinner baffles is better at attenuating mid- to high-frequency noise. The more the attenuation required by a silencer of a given cross-sectional area (square feet [square meters]) and airflow volume (cubic feet per minute [liters per second]), the greater the pressure loss (inches of water [pascal]). The key to applying ventilation silencers is to adhere to the external pressure loss allowed for the equipment. It is important to be versed in the operation of mechanical equipment and to know the available pressure and design airflow volume of a fan, upblast chiller, or induced-draft cooling tower. Blindly applying silencers will yield unwanted results, including poor operation, voiding the equipment manufacturer’s warranty.
For durability, it is best to incorporate 16-gauge G90 galvanized-steel or 18-gauge 304 or 316 stainless-steel outer shells. The inner perforated baffle skins should be constructed of at least 22-gauge galvanized or stainless steel. The acoustic-grade media under compression should be protected using a fiberglass-cloth liner or bag. For environmental (outdoor) applications, a biaxially-oriented-polyethylene-terephthalate (BoPET) film or polyvinyl-fluoride (PVF) film is not recommended, as, when these liners are used to encapsulate acoustic-grade media in silencers, double-walled panels, and louvers, the slightest tear will result in a path for water to enter. There are many occasions for this to occur post-installation. One is that these products are not ultraviolet-resistant and can degrade over time, becoming brittle. Tears and holes cause bagged fill assemblies to become like water balloons. In cold climates, water in these “balloons” freezes and, as it turns to ice, expands. The expansion damages silencers and acoustic louver baffles and panels, resulting in unrepairable damage. For silencers, fiberglass cloth is recommended because it is porous, which allows it to breathe and hold up to freeze/thaw and hot/cold environmental cycles. Additionally, it does not degrade acoustical performance. For double-walled acoustic-barrier panels and acoustic louvers, no protective film is needed. It is best to use an acoustic media that is resistant to, and will not sag under, freeze/thaw cycles.
Fixed-blade acoustic louvers. Fixed-blade acoustic louvers generally are used where low-airflow face velocities (feet per minute [meters per second])—that is, reasonable volumetric airflows and large ventilation openings resulting in large cross-sectional areas—are encountered. Simply, they can be thought of as silencers with small percent-open areas offering maximum noise reduction in a short length. They are available in 6-in. (15 cm) to 24-in. (61 cm) thicknesses and various percent-open-area designs. Such systems include louvered skirts along air-cooled-chiller barrier walls or equipment-yard barriers and intake and discharge ventilation openings for pump rooms, generator rooms, and central utility plants (Photo E). The materials of construction depend on the process. Wastewater-treatment facilities tend to require stainless steel, whereas commercial buildings require galvanized or aluminum sheet, sometimes, such as the case of architectural applications, with a powder-coated/wet-painted finish.
Double-walled barriers and enclosure panels. Barrier-wall and enclosure systems must exhibit three characteristics:
- A “critical” minimum density.
- The ability to block transmitted sound.
- The ability to absorb reflected sound.
Commonly, architects and owners use poured-concrete or concrete-block walls for barriers and enclosures. A dense material, concrete is a better sound-transmission blocker than reflected-sound absorber, so barriers made from concrete reflect or redirect, rather than absorb, noise, while concrete enclosures create a large reverberant chamber, magnifying noise instead of absorbing sound.
Light-gauge corrugated-metal sheeting also is used to construct barriers and enclosures. With densities far from what is considered “critical,” these systems are transparent to noise; as a result, noise propagates through them.
Noise-control pergolas/stacks. A noise-control pergola (Photo F) or stack (Photo G) is designed to perform like a silencer, offering noise reduction and very low pressure loss. A common application is an air-cooled chiller located in the alcove of a high-rise office building with noise propagating upward and breaking into windows or encroaching upon balconies. In such cases, a noise-control pergola can be used in conjunction with a double-walled barrier with fixed-blade-acoustic-louver skirt. When a concrete equipment-yard wall already is installed, a noise-control pergola can be used in combination with rigid or quilted absorbers to control reflective and direct-line-of-sight mechanical-equipment noise.
Sound absorbers. Often, equipment yards are constructed of cinder block or precast concrete. These hard surfaces create a reverberant sound field that magnifies and redirects noise. Sound absorbers soften wall surfaces and reduce adverse reverberant effect. Panels can be rigid (Photo H) or quilted (Photo I).
Another consideration is the design of the structural support for a noise-control pergola or stack. Mechanical equipment is not designed to accept additional dead loads or moment forces. Any noise-control solution needs to be supported externally and separately from mechanical equipment.
Proper application of noise-control algorithms and products backed by independent performance testing will achieve the average noise reductions and pressure drops in Table 1.
PHOTO C. Air-cooled-chiller sound wall skirted with acoustic louvers. Photo courtesy of Kinetics Noise Control Inc.
PHOTO D. Induced-draft cooling towers with rectangular intake and discharge silencers. Photo courtesy of Kinetics Noise Control Inc.
PHOTO E. Central utility plant with fixed-blade acoustic louvers. Photo courtesy of Kinetics Noise Control Inc.
PHOTO F. Noise-control pergola. Image courtesy of Kinetics Noise Control Inc.
PHOTO G. Noise-control stack. Image courtesy of Kinetics Noise Control Inc.
PHOTO H. Rigid perforated sound absorbers. Photo courtesy of Kinetics Noise Control Inc.
PHOTO I. Quilted sound absorbers. Photo courtesy of Kinetics Noise Control Inc.
Environmental-noise control is of great importance today. Using the most up-to-date acoustic algorithms and design standards and applying independently tested products (ventilation silencers, fixed-blade acoustic louvers, double-walled barriers and enclosure panels, noise-control pergolas/stacks, sound absorbers) will yield solutions satisfying to both the building owner and the surrounding community.
TABLE 1. Obtainable performance for various noise-control systems.
- Nriagu, J.O. (Ed.). (2011). Encyclopedia of environmental health. Rochester, NY: Elsevier.
- Murphy, E., & King, E.A. (2014). Environmental noise pollution: Noise mapping, public health, and policy (ch. 4). Rochester, NY: Elsevier.
- WHO Regional Office for Europe. (2018). Environmental noise guidelines for the European region. Copenhagen, Denmark: World Health Organization. Retrieved from https://bit.ly/Europe_Noise
- WHO. (2022). Compendium of WHO and other UN guidance on health and environment, 2022 update (ch. 11). Geneva: World Health Organization. Retrieved from https://bit.ly/WHO_Noise
- Suter, A.H. (1991). Noise and its effects. Report prepared for Administrative Conference of the United States. Retrieved from https://bit.ly/Suter_Noise
About the Author
John Sofra is director of sales, North America, for Kinetics Noise Control Inc. He has served on numerous ASHRAE technical committees for acoustics and vibration and taught continuing-education courses on ventilation and acoustics at the University of Wisconsin-Madison.