Insulation materials have traditionally been applied to the interior or exterior of duct for a combination of purposes, including thermal efficiency, condensation control, and noise control. Of these, effective noise control is by far the most challenging. It is important to remember that a conventional duct liner and, to an even lesser extent, duct wrap, will only resolve a portion of the noise issues in any building. An effective noise attenuation strategy begins at the design stage and utilizes a combination of methods including mechanical layout, vibration isolation, and duct insulation.
Types of Noise Associated with Duct
The standard definition of noise, which is any unwanted sound, is a deceptively simple summation of a complex issue that can have a tremendous impact on our living, work and learning environments. Productivity, health and learning are affected by noise and to effectively amend noise problems in buildings, and specifically the noise that is carried through ductwork, we must first understand the nature of noise and how it travels. Acousticians like to talk about sound in terms of two categories: structure-borne sound and airborne sound. The latter, airborne sound, is the noise we actually hear. A simple example of this is the noise generated from a radio. Structure-borne sound results from a physical vibration of materials caused by some impact event or other form of mechanical excitation (e.g. a hammer striking the wall). However, the noise we hear when a hammer strikes the wall is actually the airborne sound that results from a structure borne sound (vibration). In most cases, structure-borne sound travels through the building structure via construction materials, frame, and interior elements.
Consider a sheet metal duct, which is a virtual wind tunnel through which any noise can travel. It provides a structural and an airborne path for noise generated by mechanical equipment like fans and chillers. It can also transmit noise from virtually any other source in the building, e.g. people, speakers and machines. This airborne-to-structure-borne conversion can repeat multiple times until the sound source is switched off. This is why good acoustic engineering in a building requires an integrated noise control strategy. Commonly used fiber-based duct liners absorb sound primarily through viscous and frictional losses of the air oscillating inside the material. Noise is reduced because of frictional effects inside the material. These materials, in typical 1-inch thicknesses, are effective over a relatively broad range of frequencies, but their sound absorption performances diminish significantly at 500 Hz and below. Thus, when low frequency noise is anticipated as a problem, greater thickness (and length) of fibrous material is required or else the designed sound attenuation will not be achieved. Elastomeric foam products, such as ArmaFlex® and CoilFlex®, react to sound quite differently. Unlike fiberglass, which allows sound to freely enter into the spaces between the fibers, closed- cell elastomeric foam is too highly resistive to enable the viscous friction of air inside to develop an effective absorption mechanism. However, instead of simply reflecting the sound like many other types of rigid foam materials, the physical properties of elastomeric foams are such that their structure mechanically responds to the incident sound.
Solutions for Noise Control
- Non-fibrous, erosion-resistant structure that withstands velocities of 10,000 ft/min.
- Thermal properties, which meet the requirements of ASHRAE 90.1 and the International Energy Conservation Code.
- Flame spread index of less than 25 and a smoke developed index of less than 50 for all thicknesses up to and including 2" when tested according to ASTM E 84.
- Built-in EPA-registered Microban® antimicrobial protection to inhibit the spread of mold inside the ducts.
- GREENGUARD Gold certification for low emitting products.
- Low VOC, formaldehyde-free, and nonparticulating.
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