2. Origin and prediction of aerodynamic noise

Aerodynamic noise from fans is the result of interaction mechanisms between the flow and the fixed and rotating parts of the fan. At this point, it should be pointed out that aerodynamic noise occurs on all three propagation paths: airborne, structure-borne and duct-borne. The level of structure-borne noise, however, is often of the second order compared with airborne and duct noise. Furthermore, the mechanisms described below do not in any way involve blade vibrations, as these hardly ever play a part in the noise generated by fans and aerial rotating machines in general.