The mean spectral coherence obtained by averaging the entire carrier frequency f c range is shown in Figure 4b. The dominant spectral coherence (SC) values were located between 10 kHz and 40 kHz in the carrier frequency f c corresponding to the ship’s propeller cavitation noise and around 50 kHz in carrier frequency f c corresponding to echo sounder signals. The two-dimensional spectral coherence distribution as a function of cyclic frequency ( α) and carrier frequency ( f c) from the analysis of the recorded data is shown in Figure 4a. The spectral coherence analysis of the ship-radiated sound enabled both the propeller blade pass frequency B P F and shaft frequency S F to be determined, the ratio of which was the blade number. Three different approaches were applied to the POAWRS dataset in for the simultaneous and automatic detection, bearing-time estimation, and acoustic signature characterization of multiple ships over long ranges, which included broadband energetics, temporal coherence for tonal extraction, and spectral coherence for cyclostationary propeller cavitation noise analysis. The analysis was focused on the detection and characterization of ship tonal signals, as well as passive ship localization in. A large aperture, densely-sampled coherent hydrophone array with 160-elements was employed to analyze the underwater sound radiated from multiple ships using the Passive Ocean Acoustic Waveguide Remote Sensing (POAWRS) technique over instantaneous wide areas, with >100 km diameters, in several continental shelf environments. In, ship cavitation noise was analyzed using the Stevens Passive Acoustic Detection System (SPADES) with two crossed pairs of hydrophones deployed near a harbour. Other sensing modalities that deploy more than a single hydrophone have been implemented. Underwater ship-radiated sound analyses in the published literature are often based on single hydrophone measurements. Finally, an analysis of one special measurement is provided when the ship’s speed, propeller pitch, and RPM are varied over the duration of the measurement. The efficacy and robustness of the ship parameter estimation at different pitches are discussed. The 50% propeller pitch ratio was found to be a crucial point, at which multiple features of ship noise attain either their peak or minimum values. These features are then compared for different propeller pitch ratios ranging from 20% to 82% at a fixed RPM. The calculations are initially made on measurements with ship at fixed propeller pitch ratio and revolutions per minute (RPM). The frequency variations of frequency-modulated (FM) tonal signals that are probably due to the non-uniform wake from the propeller blades caused by the ship’s physical motions, such as yaw, roll or pitch, are also quantified. and includes the calculation of (i) the power spectral density (PSD) for broadband sound energetics, (ii) the temporal coherence for machinery tonal sound, and (iii) the spectral coherence for amplitude-modulated propeller cavitation noise. The ship-radiated underwater sound characterization follows the approach of Refs. Here, we analyze and characterize the underwater sound radiated by a specific ship with a controllable pitch propeller (CPP) and received on three vertically deployed hydrophones. The findings here elucidate the effects of pitch variation on underwater sound radiated by ships with controllable pitch propellers and has applications in ship design and underwater noise mitigation. The 50% pitch is found to be a crucial point for this ship about which tonal characteristics of its underwater radiated sound attain their peak values, while broadband sound and associated spectral coherences are at a minimum. Finally, analysis of one special measurement is provided, when ship changes speed, propeller pitch and RPM over the duration of the measurement. The efficacy and robustness of ship parameter estimation at different pitches are discussed. These characteristics are compared for different propeller pitch ratios ranging from 20% to 82% at fixed propeller revolutions per minute (RPM). Frequency-modulated (FM) tonal signals are also characterized in terms of their frequency variations. Here the underwater sound radiated by a ship equipped with a controllable pitch propeller (CPP) is analyzed and quantified via its (i) power spectral density for signal energetics, (ii) temporal coherence for machinery tonal sound, and (iii) spectral coherence for propeller amplitude-modulated cavitation noise. The time-dependent spectral characteristics of underwater sound radiated by an ocean vessel has complex dependencies on ship machinery, propeller dynamics, hydrodynamics of ship exhaust and motion, as well as ship board activities.
0 Comments
Leave a Reply. |