EVALUATION OF MEASUREMENT UNCERTAINTIES IN THE ASSESSMENT OF THE NOISE IMPACT OF SHIPS

Measurement operations are all inevitably affected by uncertainty, that is defined as the "degree of uncertainty" with which the measurement process obtains the result. It produces a range of values in which the true value of the measurand is present with a certain probability. In the specific case of the evaluation of the acoustic emissions associated with ships, this degree of uncertainty is referable to several causes, better understandable if the measurement procedures indi cated by the reference standards are described (for example ASA (Acoustical Society of America) S12.64-2009/PART 1), adopted for all types of ship (with no limitation in size) that transits at a speed not exceeding 50 kn in deep water. The standard offers three degrees of measurement (A, B or C), which differ in terms of uncertainty, complexity and repeatability. In general, information is given on the characteristics of the instru ments, signal processing and positioning of hydrophones. The standard is valid only in deep water and this represents its main limit: in the case, for example, of very large ships it is difficult to reach the great depths with the instruments. From the analysis of the prescriptions, it clearly follows that noise measurements in water are intrin sically affected by measurement errors that can be reported to the following three macro-types: 1. Error due to the degree of precision of the measurement chain 2. Error due to relative positioning between source and receivers 3. Error due to the characteristics of the measurement environment The 28th International Congress on Sound and Vibration (ICSV28), 24-28 July 2022 2 In the present work, all these possible sources of error have been evaluated on the basis of numerical simulation models whose results have been validated by experimental tests at sea, to arrive at the defi nition of a procedure for estimating the overall error inherent in the measurement and thus defining procedures for the control of the same.