Combined active noise and vibration control for a ventilation duct

This paper presents theoretical and experimental studies on a combination of active noise control and active vibration control for a ventilation duct. Ventilation systems radiate noise at the duct outlets and from the duct walls. In many practical applications, ventilation ducts are installed without shielding, although, they may run through corridors and adjacent rooms or halls to be ventilated. Thus both air-and structure-borne disturbances can be a cause of acoustic discomfort for the occupants. In this study the combined active control of air-and structure-borne noise is examined. In the theoretical study a duct segment with circular cross-section has been modelled using an elemental approach with coupled acoustic impedance and structural mobility functions. This model is used to investigate the principle effects and limitations of feed forward control using acoustic sources acting on the duct interior and the effect of feedback control forces acting on the duct walls. The control performance is evaluated in terms of (a) the far field sound power radiated from the duct outlet and (b) the far field sound power radiated from the duct walls. For the experimental studies a laboratory duct set-up is used, which is equipped with loudspeaker, shaker and an axial ventilator as primary sources and a combined active noise and active vibration control system. Results for loudspeaker and shaker primary excitations are in reasonable agreement with the simulations for ideal air and structure borne sources. Experiments with the ventilator source are used to investigate the system limitations in the presence of air flow induced turbulences and structure borne disturbances. Both the theoretical and experimental results show that the air borne noise radiated from the duct outlet can be significantly attenuated using feed-forward active noise control. In presence of structure borne noise the performance of the active noise control is impaired and sound radiation from the duct walls is increased. In this case, if the active noise control is combined with a vibration control system, the control of the sound radiation by the duct outlet is improved and considerable reductions of the sound radiation from the duct wall are achieved.