Fan noise visualization and noise source identification by using nearfield acoustical holography.

A nearfield acoustical holography (NAH) method combined with a non‐stationary source compensation procedure is applied to identify the noise sources of two open fans. The measured noise data of a low‐speed, small fan indicate the highest peak levels at the blade passing frequencies (BPFs) of 102 and 204 Hz. It is observed that there is no noise component induced by vortex shedding at the blade tips, or leading or trailing edges of the small fan. It is also observed that the structural resonance frequencies of the small fan blades are not coincident with the BPFs: thus, the structural vibration responses of the blades contribute insignificantly to the fan noise. The NAH results of the small fan show acoustic pressure fluctuations on the blade surfaces at the BPFs. The measured noise data of a high‐speed, large fan show the frequency components associated with BPFs as well as vortices shed at blade edges. Similar to the small fan case, the resonance frequencies of the fan blades cannot be observed in the measured acoustic data. The NAH results of the high‐speed, large fan successfully exhibit acoustic pressure fluctuations at BPFs as well as noise patterns radiated from the vortex shedding.