The sources of trans-spectral coherence in the radiation from rotating blades

Rotating blades (installed in fans, propellers, and helicopters) radiate both discrete and broadband sound. This radiation can be examined for the coherence of the trans-phase. Trans-phase is the phase difference between the components at two different frequencies when one of them has been sent through a nonlinear filter. If the trans-phase shows coherence, the signals are said to be trans-spectrally coherent. Various sources of the rotating machinery noises contribute to the coherence in different ways. Existence of such coherence can help in the diagnostics and identification of the sources and in the detection and classification of the machinery in presence of extraneous noise.