Duct insulation also helps to reduce noise in the ductwork. This noise is from two sources: (1) aerodynamic noise generated by the flow of air through elbows, turning vanes, takeoffs, and other components of the ductwork, and (2) noise traveling through the duct from the source. Breakout noise is that portion of the duct noise that escapes into the occupied space.
The highest noise level in a supply air system happens just after the fan. There are several effective ways to reduce noise at this point. A rooftop DDX unit, for example, with a short branch run to the diffusers might have a 90° elbow in the duct run to minimize the noise reaching the diffusers. The return duct will often need the same extra turn, to prevent noise from being transmitted from the rooftop unit via that same route.
Air handlers in a mechanical room near the zones might take a different approach, since a radical turn in the supply duct just after the fan may cause excess static pressure. An inexpensive method is to wrap the duct in two layers of gypboard for 10 to 20 ft downstream of the fan. An improvement on this is to trap 1 or 2 in of insulation between the gypboard layers. This keeps the layers out of direct contact and prevents vibrations from being transmitted to the outer layer of the assembly. If this happens, there will be little reduction in noise transmitted.
A duct noise silencer is a proper solution in other circumstances, such as in a concert hall or hospital where noise levels must be minimized. The silencer should be located in supply duct upstream of the occupied spaces and downstream of them in return ductwork.
These principles should be kept in mind whenever considering energy- conservation projects that involve ductwork so that energy is not saved at the cost of increased noise or mechanical vibration.
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Excerpt from Energy Conservation Projects.