To elaborate just a tad on this point, keep in mind the way that sound waves behave. If you have a set of sound waves in phase, their amplitude (i.e. volume) multiplies at those intersecting points. A series of waves completely out of phase essentially cancels the net resulting energy, thus producing no volume (this is how noise canceling headphones work, btw). By having a prime number of blades, the sound waves coming off the fan blades will never have the opportunity to be in phase and not have a net resulting amplitude higher than the initial blade/air compression (at least in theory at a constant speed). By altering the spacing I would imagine this creates even more of an out-of-phase condition and more easily adapts to the changing speed of the engine. You should be able to have non-prime blade count with spacing that still allows for an out-of-phase condition to occur.

Edit: Actually I'm wrong on the prime part. Having them evenly spaced would create the same exact wavelength between each blade (same pitch and depth) and therefore amplify the noise dramatically. Having them unevenly spaced creates a different amount of length between each pulse, making for different wavelengths at each blade, each being out of phase with one another. Sophomoric physics mistake on my part but hey, I'm not an engineer...

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth