More often than not noise transfers via ductwork – whether it be in a residence, an industrial facility, or a commercial office building. Sometimes the noise is from the HVAC system itself and other times the noise travels from vent to vent (ie. One person is talking in an office that has shared ductwork with other offices and the other offices can hear the conversations).
Regardless of where the noise originates, the sound travels through the ductwork in the same way. Because the ducts are typically made from sheet metal and are formed into a rectangular shape, this makes the perfect reflective passageway for noise to travel. This is especially true if the noise has no bends or corners to travel around.
The best way to handle this is to design the original ductwork with as many corners and bends as possible. Additionally, adding a soft lining, such as foam or fabric will reduce noise. By designing it this way, the bends and corners will increase the distance that the noise has to travel and some of the reflections will be re-directed in the opposite direction. The soft lining will dramatically reduce the sheet metals ability to reflect the noise at all.
With a combination of these two things the sound transmission will be dramatically reduced, resulting in much more private offices and quieter living environments. Of course, every scenario is different and the sound transmission is entirely dependent upon the frequency content of the original sound, the mounting system of the ductwork, the sizing of the ducts, and the angles that the ducts are mounted. Without the proper engineering, adequate results may not be obtained and in some cases, it might become even worse. Contact a professional at Memtech Acoustical to assure optimal results.
Brendon Doran, Acoustical Consultant