Complex demodulation of broadband cavitation noise.

Abstract

Cavitation is a prominent source of ship noise resulting from the collapse of bubbles in the wake of rotating propeller blades. This same rotation also induces an audible rhythm, which is often modeled as a periodiclike modulator multiplying a broadband noise carrier signal. Conventional signal models restrict the modulator term to be non-negative and real-valued, accounting only for long-term energetic fluctuations in the time series data. However, recent empirical observations [Versluis et al., Science, 2000] and explanatory simulations [Ida, Phys. Rev. (2009)] reveal that interactions between multiple bubbles can lead to phase inversions in the form of negative pressure spikes. In this context we propose a more general complex-valued modulator that can account for energetic as well as phase-coherence patterns in the signal model. Through the use of synthetic signals and ship data examples, we will briefly discuss issues of detection and maximum-likelihood estimation related to the new complex signal model. For the purpose of fostering scientific understanding, however, our primary focus will be motivation for and estimation of the significance of possible complex modulation in both modeled and observed cavitation noise. [This research was supported by the Office of Naval Research.]