Spectral analysis of the underwater acoustic noise radiated by ships with controllable pitch propellers F. Traverso, T. Gaggero DITEN, University of Genoa Genoa, Italy federico.traverso@ginevra.dibe.unige.it E. Rizzuto DICCA, University of Genoa Genoa, Italy A.Trucco 1,2 1 DITEN, University of Genoa 2 Istituto Italiano di Tecnologia (IIT) Genoa, Italy Abstract—This paper presents spectral analyses of the underwater noise radiated by ships equipped with controllable pitch propellers. The noise measurements were performed at sea for three different ship’s types. Each ship passage was characterized by a specific combination of propeller rotational speed and propeller pitch, allowing to investigate the variations of the radiated noise spectrum at different settings of the pitch- RPM combination law. Neglecting the low-frequency contributions, the focus is on the analysis of the broadband patterns in the range 100 Hz to 4 kHz A parameterization derived from a basic spectral pattern is adopted: the noise spectrum shape is approximated with a flat level magnitude up to a reasonable frequency followed by a logarithmic decay. An investigation of the relationship between the considered parameters and the actual ship condition is proposed along with a comparison with the predictions provided by classic Ross and Whales-Heitmeyer ship noise models. Keywords—spectral analysis, underwater ship noise measurements, controllable pitch propeller. I. INTRODUCTION In the last decade of the 20th century the continuously increasing human activities in the oceans has focused the attention of local and international institutions on the underwater noise pollution and, among other anthropogenic sources, on shipping traffic in particular. For military purposes the topic of underwater ship noise radiation had a fundamental role since the WWII, but, in the civil field, only recently, studies of the radiated noise have been promoted in order to reduce the shipping noise impact on the marine fauna [1]. To assess such impact, the identification of the key characteristics of the ship noise spectrum even with simplified patterns represent a significant target. Furthermore, the underwater noise plays a fundamental role in defining the characteristics of the channel for acoustic communication [2],[3]. When present, ship noise is the major contributor in its frequency region [4]; it follows that a knowledge of its spectral characteristics is necessary for those systems designed to work in the same band [5]. Through the years several ship noise data set have been published and spectral analysis have been performed to investigate the major contributors to the acoustic spectrum [6]- [9]. Often, the measurements were performed following different strategies and possibly presented with ad-hoc metrics. It follows that difficulties arise when comparing data coming from various sources. Moreover, so far, there is not a unique, widely accepted standard for underwater measurements even if in the last years national and internationals bodies have proposed guidelines on the subject [10]-[13]. One of the aims of measurements may be to investigate the spectral content of the radiated noise in order to retrieve synthetic information about the ship characteristics: e.g. class, speed of advance, displacement, etc.. The outcome of the analysis, establishing a relationship between the radiated noise spectrum and the ship data, may be used conversely to reproduce the observed spectral behavior and therefore to predict the ship noise for a variety of possible characteristics or operating conditions of the ship. This can be accomplished with simplified spectral shapes able to capture the main patterns of the underwater noise radiation. The ship radiated noise is generated by several sources on board: machinery- generated hull vibrations, the rotating pressure field of propellers, cavitation phenomena on the blades, turbulence and vortices in the flow around the hull, etc.. In the frequency range from a few tens hertz to several kilo hertz it is mainly dominated by broadband components like those of hydrodynamic flow and propeller cavitation. Within this range the total acoustic noise exhibits a continuous spectrum that peaks at a given frequency [6],[14]. It follows that equations are often required to tune a spectral shape with a flat level magnitude up to a reasonable frequency (e.g., 100 Hz) followed by a logarithmic decay (e.g., 6 dB per octave) [7],[15],[16]. In the literature several empirical models were proposed to estimate the ship noise taking into account macro factors like speed and displacement [7]. Further, models have been obtained either by aggregating ship noise spectra over a specific ship class [17] or by considering reference parameters for a single vessel [15],[16]. Most of the models however, are based on acoustic measurements concerning ships equipped with fixed pitch propellers (FPPs), while many modern ships feature controllable pitch propellers (CPPs). As known, the control of pitch is realized with a rigid rotation of the blade around a radial axis, thus implying an equal modification of the orientation of the blade profiles at the different radii. Such orientation in design conditions is different for the various blade sections, so a rigid rotation generates a different variation of the hydrodynamic load, initially regularly distributed along the radial direction. The final result is to have sections at different radii working in hydrodynamic conditions different 978-1-4799-8736-8/15/$31.00 ©2015 IEEE This is a DRAFT. As such it may not be cited in other works. The citable Proceedings of the Conference will be published in IEEE Xplore shortly after the conclusion of the conference.