Noise suppression of a dipole source by tensioned membrane housing

Reducing the ducted-fan noise at the low frequency range remains a big technical challenge. This study presents a passive approach to directly suppress the sound radiation from an axial-flow fan of dipole nature housed by a tensioned membrane backed with a cavity. The use of the membrane not only eliminates the aerodynamic loss of flow, but also provides flexibility in controlling the range of the stopband with high insertion loss by varying its tension and mass. A theoretical model is presented which allows the performance of the proposed device to be explored analytically. With the proper design, this new device can achieve better noise reduction than the empty expansion cavity. Through the detail modal analysis, even in-vacuo modes of the membrane vibration are found to play an important role on the suppression of sound radiation from dipole source.