The three types of fan noise shown in this slide are directly caused by differences in air flow. The sound generation due to uniform and steady aerodynamic force has very weak noise for low speed fans, and depends on fan RPM. This kind of noise is impossible to avoid, although it can be reduced by increasing the number of blades. Steady but non-uniform inlet air flow causes non-uniform aerodynamic forces on the blades as the angular positions change. This is one of the major noise sources of electronic cooling fans. Unsteady flow causes random forces on the blades which spreads the noises created and makes them continuous.