Fan noise isn't even remotely periodic, because most of it is due to moving air rather than moving blades.
However, you can take advantage of that because the sounds you want to keep generally are periodic at short timescales. You can use DSP filters to remove components of the sound that don't show significant autocorrelation. The same idea lets you remove white noise and random static from radio signals.
However, you can take advantage of that because the sounds you want to keep generally are periodic at short timescales. You can use DSP filters to remove components of the sound that don't show significant autocorrelation. The same idea lets you remove white noise and random static from radio signals.