Noise characteristics in extreme shallow water with relation to ship maneuvering parameters

摘要

This paper examines the propeller characteristics of the radiated noise from a small research fishery vessel. In contrast to the typical ship noise analysis, where standards are provided for measurements in deep and relatively shallow water, the measurement in this work was conducted in extreme shallow water (depth less than 20 meters). Two recording systems (two hydrophones each) were used for acquiring the data, and a test vessel was utilized to perform several types of maneuvering lines. This study addresses the extraction of the ship propeller characteristics such as the rotation speed, the number of blades, and the propeller acceleration. Demodulation technique of the broadband propeller cavitation noise was implemented in two forms. In the uniform rotation of the propeller, the classical Fourier transformation (FFT) was utilized to analyze the envelope of the cavitation band and to extract the periodic characteristics of the propeller. In a non-uniform rotation behavior, the fractional Fourier transformation (FrFT) was used for extracting the instantaneous frequency information. In both methods multi-band filtering technique was implemented and showed improved peak to noise ratio for better extraction result. In addition, the radiated noise level (RNL) was estimated using two different propagation models, the cylindrical spreading loss model and BELLHOP-3D model that is based on ray tracing. The comparison of the variability of the estimated RNL using both propagation models showed high sensitivity of the BELLHOP-3D model to the distance between the target and the source.