Concerning the fan system with the long pipelines connected on the inlet and outlet, the structural vibration is the main component of the external radiated noise and is caused by the volute vibration which is excited by the unsteady aerodynamic force on the volute casing. The main conclusions obtained are as follows:
1) In this paper, a method of numerical calculation of air-inlet-sound is to be implemented, and the rationality of this method is to be verified by dynamic pressure measurement and vibration measurement of the volute casing as well as fan bearings including temperature
2) The normal vibration of the volute was the decisive factor that determined the volute surface  vibration.