Using the 9G fan model shown here with 9 blades per impeller and spinning at 11,000 rpm, the engineer can now calculate the frequency of wind noise. Start by converting the rotational speed of the fan from the RPM into Hertz, where 11,000 rpm equals 183 Hz. Multiply this by the 9 blades, and one can expect a peak from wind noise to occur at 1,647 Hz, as shown here in the bar chart of the recorded sound pressure per frequency band.