I wasn't alluding to using X number of band pass filters; just a single notch (band stop) filter. The notch removes the fundamental frequency that would be present when the device is rotating correctly, and the output level would be of low amplitude. When something goes wrong (blade breaks), the notch would not remove all the other frequency components created (and the 'fundamental' would likely be outside the notch bandwidth), therefore the filter output amplitude would be higher.