In HVAC systems, ducts distribute more than air; they also distribute noise produced by fans. Duct liners (Photo A) were developed in part to absorb noise before it emanates from ductwork. Designers apply different thicknesses and lengths of duct liner to reduce noise to various levels. Understanding the acoustical benefits of duct liners and the available choices in liner materials helps in specifying a system that is efficient and effective, both in terms of noise performance and HVAC performance.

The Buzz on Noise
Sound is a rapidly repeated fluctuation in air pressure. It is energy in motion, kinetic energy in the form of air pressure. If the frequency of change is consistent, we may perceive sound of a particular musical pitch. When pressure changes happen at random frequencies, the sound is called "broadband," which often is perceived as "noise."

The percentage of change in ambient atmospheric pressure needed to create audible sound is small. Pressure fluctuations of less than 3 percent, occurring at a frequency of hundreds or thousands of times per second (audio frequencies), are perceived by the human ear as painfully loud sound.

Fans capable of moving air through an entire building are powerful, generating a significant amount of energy in ductwork, some of it in the form of noise. This energy is distributed throughout the duct system until it is either allowed to escape through openings to the building or absorbed.

Fan noise can create acoustic conditions that are undesirable and unacceptable for many types of occupancy. It might interfere with human speech, for example. However, a low level of background noise often is considered beneficial for ordinary activities, such as those in an office, because it improves speech privacy by interfering with the audibility of more distracting noises, such as a talkative colleague.

Soaking up Noise
Duct liners remove noise energy from ductwork without significantly reducing the overall kinetic energy that has been put into the system to move air. They do this by acting as sound-absorbing surfaces on the inside of air ducts, reducing the transmission of noise. Duct liners take noise energy out of a system by transforming it into another form of energy, usually heat.

There are two basic types of duct liner: fibrous and foam. Fibrous and foam duct liners absorb noise in slightly different ways.

Fibrous liners are made of unconnected, loosely packed glass fibers. As air passes through the bundle, the fibers rub against one another, which produces infinitesimally small amounts of heat. Sound is absorbed through this transformation of kinetic energy into friction.

Polyimide is a flexible open-cell foam. Polyimide-foam liners have a contiguous, as opposed to open, network structure, but behave similarly to fibrous liners in that sound energy enters, is converted to heat, and dissipated.

Elastomeric foam generally is a closed-cell material prohibiting air penetration. Elastomeric-foam liners are made of an expanded cross-linked rubber compound, the cells of which are fully enclosed gas bubbles. Air pressure impacts the surface of the foam, compressing it slightly. In the process, acoustical energy is absorbed.

Figure 1 compares the sound-absorption efficiencies of fiber-glass, polyimide-foam, and elastomeric-foam duct liners. Testing was performed per ASTM C423, Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method.

Sound absorption is directly proportional to exposed surface area and liner thickness. Based on those factors, a designer can compute the length of ductwork that would have to be lined to achieve a given level of noise reduction. For example, consider a project in need of 5 dB of fan-noise attenuation at 250 Hz through 24-in.-by-24-in. ductwork. Using the path-noise estimation methodology presented in Chapter 48 of 2011 ASHRAE Handbook—HVAC Applications,¹ approximately 50 ft of unlined sheet-metal ductwork would be needed; alternatively, this attenuation can be provided within a length of only 10 ft of ductwork with 1-in. sound-absorbing internal lining (which achieves a noise-reduction coefficient [NRC] of 0.70 or higher, as measured by ASTM C423).

Internal liners reduce duct cross-sectional area. For that reason, liners usually are kept to a minimum thickness, often 1 in. To illustrate the impact of liner thickness, for an 18-in.-by-24-in. duct, going from a 1-in.-thick liner to a 2-in.-thick liner decreases flow area by about 20 percent, resulting in an increase in air velocity of 26 percent.

Designing Noise Levels
Designing mechanical-system noise levels involves consideration of the following factors:

• The target noise goal.
• Air-handler noise output.
• Liner-material noise-reduction capability—measured as sound-absorption coefficient/NRC or "insertion loss."
• Duct dimensions.

Noise goals depend on the use of a space. An office, apartment, or classroom usually requires a moderate amount of HVAC-noise reduction for normal activities to be accommodated. For these projects, as little as 10 to 20 ft of lined duct, usually immediately adjoining the mechanical room, is all that is needed for noise goals to be met.

Conversely, certain spaces are more sensitive and require low mechanical-noise levels—performance spaces and recording studios are two examples. The ducts of such spaces may need to be lined continuously from one end to the other.

Acousticians establish specific noise goals using a measure called room criterion (RC), the overall background-noise level across the audible frequency range. The RC for a generic office space might be 30 to 35, while for an airport lobby, it might be about 40, and for an extremely quiet concert hall with exacting acoustical demands, it might be less than 15. For this target level to be achieved, all sources of noise, not just the HVAC system, must be considered.

The noise output of air handlers generally is specified by manufacturers per Air Movement and Control Association standards.

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