Quiet Comfort

04/30/2003 |

HVAC Systems and Acoustics

Motor and fan noise from HVAC equipment is transmitted by bare sheet metal ducts. Lined ducts, like the one being attached here, significantly reduce noise transmission.

In today’s competitive economy, worker productivity is critical to success. A great deal of new technology is devoted to improving productivity, but one of the simplest and most obvious factors is often overlooked – noise. According to the American Society of Interior Designers (ASID), Washington, D.C., an industry-sponsored study showed that more than 80 percent of the workers surveyed believed they would be more productive if their workspaces were quieter. Their belief was confirmed: When noise was reduced, productivity measurably improved.

In the office, constant or intermittent unwanted noise is distracting, even uncomfortable. It affects concentration and, for many people, causes fatigue. In schools, the problem is more acute, with unwanted noise actually impeding learning (see sidebar, opposite page). Optimum acoustics depends on many factors, but the HVAC system (both equipment and duct work) is often a culprit in creating and transmitting unwanted noise.

The first consideration is airflow velocity. ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers), Atlanta, recommends that air travel at velocities less than 1,500 feet per minute as it exits diffusers and grills. Higher speeds can cause disturbing hissing or whistling sounds. A frequent cause of this problem is installing ducts that are too small for the required air volume. An undersized duct acts like a nozzle, concentrating the air volume and forcing it to flow at excessive velocities. That can create constant background noise.

Another source of airflow noise is turbulence created where rectangular ducts change size or direction. Turns at 90-degree/right angles are a worst-case turbulence scenario, where the dead air spot in the back corner forms a vortex, creating an annoying air rumble. This problem is magnified as system velocity increases. Metal turning vanes reduce turbulence by smoothing airflow in the right angle, but they can create their own noise problems by vibrating and reflecting or regenerating sound.

Besides airflow noise, ducts transmit motor and fan noise from HVAC equipment. Bare sheet metal air ducts, the most common system for carrying conditioned air, provide a limited noise barrier and have no sound absorptive properties. On the contrary, bare sheet metal ducts act as conduits for equipment noise. Sheet metal also clicks and pops due to expansion and contraction from air temperature changes, and may even vibrate from airflow or fan action.

Bare or externally insulated ductwork contributes to “cross-talk,” the transmission of sound between rooms through the ducts. Hearing a neighbor’s phone conversation or worse, radio, through the air duct certainly impedes concentration and compromises privacy. We know of one law firm where the conference room shares no air ducts with any other offices in order to guarantee that no confidential conversation can be overheard through the air-handling system. This is an expensive and potentially uncomfortable way to ensure acoustic security.

Seeing the Invisible Problem

Professional acousticians and noise control engineers use many technical measures for determining the right level of background noise for a given building or room. Decibel levels vary according to the size and use of the room, but generally speaking, most offices, classrooms, and hospital rooms should have background noise no higher than around 35 dBA (A-weighted decibels). Given that HVAC systems can have airflow surges that increase sound pressure levels by up to 10 decibels, it is obvious that reducing the amount of that noise reaching occupied space is critical.

Wiring, telecommunications, traffic patterns, even air delivery routes are all clearly visible on blueprints or engineering plans. Acoustical issues and potential problem areas are not that easy to see. The planning stage is the best time to consider these problems and address them cost-effectively. It is fairly economical to design an air-handling system to minimize unwanted noise, but once the space is built, options are much more limited and solutions more costly. Adding sound control measures such as those described in the following text may add one to five percent to initial cost, but trying to solve the problem after construction is completed can easily drive up costs as much as 50 percent, particularly if legal action is involved.

Controlling Noise in Air-Handling Systems

A good initial design can go a long way in preventing unnecessary noise from the HVAC system. The Acoustical Society of America, Melville, NY, makes recommendations for classroomsii  that are equally applicable for commercial and retail space. They suggest locating rooftop mechanical equipment, VAV boxes, and fan-coil units away from critical listening spaces. For example, the unit should be placed near unoccupied space and air supply and return ducts located over a hallway or communal space instead of running directly over offices or classrooms. This greatly reduces the potential for crosstalk and equipment noise transmission (see Figure 1). Duct runs into individual rooms off the main supply and return ducts virtually eliminate noise transmission from room to room.

Beyond the initial design, mechanical engineers and facilities managers can try many things to reduce unwanted noise from HVAC systems. One is to downsize HVAC equipment and reduce airflow; in other words, use several smaller units and place them closer to occupants. This reduces the air volume per unit, cutting high air velocities. Smaller units can reduce overall equipment noise if properly placed. However, this solution must be carefully planned in advance for several reasons. First, there must be space for the units. Second, quiet equipment must be specified, especially since the equipment is now closer to building occupants. Note that the use of in-room or portable air-conditioning units often makes noise problems worse, not better, for this very reason. Furthermore, multiple units may cost more initially and result in increased maintenance costs.

Silencers are another solution. From the outside, a silencer looks like an oversized air duct. Inside are channels defined by perforated sheet metal, typically packed with dense fiber glass acoustical insulation. The fiber glass absorbs much of the equipment noise as the air passes through the flow path. The ends of the channels are tapered to reduce noisy turbulence as the air enters and exits the silencer. Silencers restrict airflow, and may require more powerful fans to maintain the desired air volume. They are also much larger and heavier than the air ducts, often requiring six to 10 feet of unoccupied space, so again advance planning is necessary. Silencers can be expensive and usually cannot be used as a remedial solution unless there is a dedicated space for them, or special provision made in the mechanical equipment room.

Active noise cancellation equipment should be viewed as a “silver bullet” – an effective solution, but only to a very specific problem. Briefly stated, noise cancellation uses a microphone to capture and analyze the offending sound wave and then send back a wave that counteracts it. This system can work well to reduce one specific, targeted source of noise, particularly at low frequencies; however, it does not treat the full sound spectrum and consequently is not effective in reducing overall noise from a variety of sources. Noise cancellation equipment requires 10 to 20 feet of space and can easily cost 10 times what a silencer costs. Nonetheless, in the right situation, it is less expensive than tearing out walls, ceilings, and plumbing to install soundproofing. Installing active noise cancellation equipment requires a trained expert, but even then success may be limited because so many factors are involved.

It also should be noted that there are no industry-recognized performance standards for silencers or noise cancellation devices.

Increasing duct size to lower air velocities effectively reduces noise in the right applications. Large diameter round ducts commonly are used for this reason in places like restaurants, community centers, convention centers, and sports complexes where ceiling space is not a problem. Round ducts often are left exposed to add visual interest. Acoustical performance of oversized and round ducts is further improved by adding a fiber glass duct liner.

The solution that can deliver significant HVAC noise control for the dollar is lining metal air ducts with fiber glass duct liner. Unlike any of the solutions already described, lined duct requires no extra space. Ducts are lined by attaching a fiber glass blanket or board material to sheet metal duct sections with an adhesive and pin fastening system. The airstream surface of these cost-effective passive acoustical products is specially treated to withstand airflow. The finished ducts are assembled in the sheet metal shop or on-site and installed the same way as bare sheet metal ducts.

Lined ducts provide significant noise reduction. A common reference for measuring sound absorption of duct liner is the Noise Reduction Coefficient (NRC), an average of the sound absorbed at four different frequencies. For example, a one-inch-thick fiber glass duct insulation from Johns Manville has an NRC of 0.70; that means it effectively absorbs 70 percent of the sound at the most common frequencies. Two-inch-thick duct liner has an NRC of 0.90, absorbing 90 percent of the noise. Fiber glass duct liner achieves this premium acoustical performance at a cost of pennies per foot.

Lined ducts have the added advantage of thermal performance, making air-handling systems more energy efficient as well as more quiet. R-value exceeds 4 per inch of most fiber glass duct liner products. Thermal and acoustical performance are documented using industry standard test methods specified under ASTM C 1071.

There are other fiber glass products that contribute to quiet, comfortable air-handling. Turning vanes made of fiber glass instead of metal eliminate the noise of turbulence and vibration at duct angles. Ducts fabricated from fiber glass duct board eliminate the need for sheet metal altogether and offer sound attenuation similar to lined duct. Duct board can “leak” some noise, but does not carry it the length of the duct like bare sheet metal. This makes duct board popular for duct plenums or sections leading directly from the HVAC equipment. Any noise leaked by the duct board is diffused into unoccupied space and quiet air is sent on into the system.

Ask an Expert

The issue of noise and productivity can be complex. Many factors play a role, especially in today’s open office plans. Acoustics should be considered from drafting right through to buying the last chair. Enlisting the advice of a professional acoustician or a noise control engineer certified by the Institute of Noise Control Engineering (INCE), Ames, IA, can save significant time and money. While the HVAC system is only part of the overall story, a quiet system can go a long way toward promoting comfort and productivity.

Eric G. Schakel is air handling product manager and Brandon Tinianov, PE is research manager at Johns Manville (www.jm.com), Denver.

Related Coverage