Multiband analysis for colored amplitude-modulated ship noise

Propeller radiated cavitation noise is broadband yet audibly rhythmic, taking on the characteristics of amplitude-modulated noise. The study of this process is important because the shaft and blade rates, as well as other identifying features of the ship, can be inferred from the cavitation signal envelope. Unlike the conventional method for estimating the modulation frequency, this paper proposes a multirate subband method applicable to colored signals, and shows the conditions under which it is optimal in the sense of maximum likelihood. Application to actual ship noise data confirms the utility of the new method, yet also reveals previously unobserved signal dynamics, namely acoustic frequency-dependent phase shifts and amplitude variation in the modulation spectrum.