AL-2503 -- Control of Low Frequency Duct-Generated Noise in Building Air Distribution Systems

One of the primary sources of rumble originates in building air-handling systems ductwork. Poor duct system design often restts in the creation of excessively turbulent flow inside the ductwork. This turbulence excites low-frequency duct-related resonances, and results in noise radiation to adjoining occupied spaces. Because of its flexibility, rectangular ductwork is more apt to cause problems than is circular ductwork.

Once generated, sound energy in the 16, 31.5, 63, and J25 Hz octave bands is not readily attenuated by "add-ons" to the system. Often, attempts to attenuate this noise with fixes such as mufflers, duct lagging, etc., make the problem worse since these devices attenuate the high frequencies without significantly affecting tpe low frequency energy. The result is an even more unbalanced sound spectra with the low frequencies dominating and the resulting noise heard as rumble.

The resolution of most of these problems depends on the use of good design practice in sizing and specification of the building ductwork. Cerami, Shatalon and Bishop (1), (2) have provided reasonable design guidelines which, if followed, tend to minimize separated flow and the resulting duct rumble. ~dditional recommended practices are also suggested by Kingsbury and Sessler (3). However, these guidelines cannot always be followed due to limitations of available space in which to mount the air handler and associated ductwork. Often a designer must compromise and expand air in a shorter distance than recommended and place elbows very close to the fan cutoff. This paper discusses these factors and suggests design guidelines for duct design and the design of mechanical equipment rooms to minimize and/or contain noise from air distribution systems.