“Apart from motor and bearing noise, most of the acoustic power [of a fan] comes from the eddy swirls following the trailing edge of the blade after it passes by. There is also an outward pulse of air as the leading edge of each blade pushes forward cutting the air.”
I found this information on the web at https://physics.stackexchange.com/questions/69915/what-part-of-the-fan-blade-actually-does-the-noise-generation, definitely worth a quick look.
The article continues: “The trailing eddies produce a broad spectrum of random noise, modulated by the fan blade frequency. The outward pulses, of course, occur at the blade frequency, with harmonics. Both are stronger near the tip, because the tip is moving faster. Faster splitting of the air means a sharper leading edge of the pressure pulse, so the higher frequency noise is especially concentrated near the tips. A small microphone near the spinning blades would pick up a repeating soft step function from the blades’ leading edges, with faster-rising step functions (thus composed of higher frequencies) farther from the hub.”
Based on this information I would climb a ladder, get as close to the blades of the fan as is safe and record with my on-camera mic or my stand-alone recorder. Either should allow you to record the sound, of which there will probably be relatively little coming from a large ceiling fan. Try the fans at various speeds to see what you get.
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