This paper summarizes the main properties of six time-frequency representations, and describes their application to the visualization of typical helicopter rotor noise traces. Time-frequency representations may be a useful tool for the visual analysis of helicopter rotor noise, because they provide information not available from time- or frequency-domain represen- tations alone, such as the time variations of the spectrum of the signal or its power spectrum, which translate in distinctive patterns on a time-frequency plot. No single representation is best for all cases. The Short-Time Fourier Transform and the spectrogram seem to have the most general usefulness. The Wigner-Ville distribution is useful when the noise events are well separated in time.
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