Low-frequency HVAC 'rumble' in a high-end condominium

Sound and vibration measurements were conducted at a high-end condominium located in downtown Washington DC.  Data was collected within the client’s residence unit and on the building’s roof. The objective for the measurements was to identify and characterize the sound within the unit for which the resident had complained. The source of the sound in the bedroom was identified from vibration on a wall adjacent to an HVAC vertical duct. The sound and vibration were found to originate at a roof-mounted fan integral to the HVAC air distribution system. The character of the sound was modulating, low-frequency ‘rumble’. Further investigation on the roof, however, revealed the building’s structure to be adequately isolated from vibration due to the fan and motor. Rather, the sound was generated by the interaction of the fan’s aerodynamics and the acoustical properties of the vertical duct. The amplitude of the acoustic field in the duct was sufficient to excite vibration of the duct’s enclosing walls. Instead of modifying the structural geometry of the vertical duct, the most efficient ‘fix’ to attenuate duct rumble was by installing a silencer in the horizontal duct between the exhaust fan and the vertical duct.