Processing tools to separate out cavitation and machinery noise

The need to isolate the contributions due to machinery noise and cavitation noise has become an important issue to be dealt with to improve the understanding of the fundamental underlying mechanisms of propagation into the sea as well as to implement effective measures to mitigate their effects on the overall radiated noise. As both machinery noise and cavitation noise are in the same frequency region, straightforward signal processing techniques are practically inadequate and advanced methods are needed for the selective detection of the different noise sources. This is the major aim of Work Package 2.4. In particular, the objective of this task is to estimate the acoustic source strength of a cavitating propeller and to separate out the contributions due to machinery noise and cavitation noise using advanced signal processing techniques, in conjunction with Task 2.5. The research work within SONIC WP2 Task 2.4 "Processing tools to separate out cavitation and machinery noise" has been carried out by: ï,· CNR-INSEAN, Italy (Task Leader) ï,· MARIN, Netherlands and, specifically, it has concerned the following topics: 1. Parametric study of noise, hull pressure fluctuations and vibrations in a scaled model of the "Princess Royal" Research Vessel (CNR INSEAN). The activity concerned the execution of a parametric study based on noise and vibration measurements on a scaled model of the "Princess Royal" Research Vessel. Aim of the survey was to collect a comprehensive database of noise, pressure fluctuation and vibration measurements to be used for verifying the methodologies developed within Work packages 2.4 and 2.5. The parametric study concerned different combinations of model excitation, propeller installation and propeller cavitation. 2. Development of an advanced signal processing technique to separate out the contributions due to machinery noise and cavitation noise (CNR INSEAN). The approach consists of a spectral conditioning technique which allows the separation of the machinery noise contribution from "in-field" hydrophone or pressure signals, by removing the part correlated to "out-of-field" sensors located in correspondence of the engine room and of the driving system. 3. Application of a method for the computation of the underwater-radiated far-field noise due to propeller cavitation to the case of the 'Princess Royal' Research Vessel, at the blade rate frequency (MARIN). The method attempts to determine the far-field noise by determining the propeller's acoustic source strength on the basis of near-field hull pressure measurements. The method is based on the inverse application of a panel method for acoustic scattering.