Wavelet analysis of blade–vortex interaction noise

Fourier analysis techniques for analyzing helicopter noise signals have been shown to be an adequate, yet inefficient tool for the design of helicopter noise control systems. Impulsive sources of noise, such as blade–vortex interaction (BVI) noise cannot be localized in time without collecting relatively large amounts of data at high sampling rates. Attempts to minimize the BVI phenomenon through helicopter blade control schemes in the Fourier domain are inherently inefficient since the control matrices must account for the large number of Fourier components needed to represent the signal. Time-frequency representations, such as the wavelet representation, offer an efficient alternative to these Fourier domain methods. Wavelet analysis has been performed on an ensemble of helicopter noise signals. An interactive wavelet program was designed using quadrature mirror filters developed by S. Mallat. Using the bandpass representations of the signal, BVI noise was isolated and extracted from the overall helicopter noise signature. Future research will integrate the wavelet decomposition into an adaptive frequency domain controller for BVI noise minimization.a)With NASA Ames Res. Ctr., MS T-42, Moffett Field, CA 94035.