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On the possibility of estimating ship parameters using acoustical Lloyd's mirror effect
No full textThe Lloyd's mirror effect is an acoustic effect taking place when a source of noise moves parallel to a reflecting surface. In the case of ships, the underwater sources of noise, mainly the propeller, are at constant depth, i.e. a constant distance, from the free surface that can be considered a perfectly reflecting surface. In such a configuration, a destructive-constructive interference pattern is generated in the time-frequency domain by the combination of the direct and surface-reflected sound waves. As the shape of the acoustic patterns depends on the speed of the ship, the relative distance between the ship and the hydrophones and the sound celerity, it is in principle possible (it has been successively done for aircrafts) to derive the above mentioned characteristics simply analysing the corresponding Lloyd's mirror effect. This is particularly interesting when monitoring shipping noise traffic when data regarding ships cannot directly derived by the Automatic Identification System (AIS). In the paper a set of measurements carried out in the framework of two EU FP7 European Projects are analysed to discuss about the possibility of estimating ship parameters using acoustical Lloyd's mirror effect. During the measurements the main ship parameters have been monitored by GPS therefore representing a good reference test
Design of Frequency-Invariant Robust Beam Patterns by the Oversteering of End-Fire Arrays
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Estimating the performance of a superdirective microphone array with a frequency-invariant response
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Maximum Constrained Directivity of Oversteered End-Fire Sensor Arrays
No full textFor linear arrays with fixed steering and an inter-element spacing smaller than one half of the wavelength, end-fire steering of a data-independent beamformer offers better directivity than broadside steering. The introduction of a lower bound on the white noise gain ensures the necessary robustness against random array errors and sensor mismatches. However, the optimum broadside performance can be obtained using a simple processing architecture, whereas the optimum end-fire performance requires a more complicated system (because complex weight coefficients are needed). In this paper, we reconsider the oversteering technique as a possible way to simplify the processing architecture of equally spaced end-fire arrays. We propose a method for computing the amount of oversteering and the related real-valued weight vector that allows the constrained directivity to be maximized for a given inter-element spacing. Moreover, we verify that the maximized oversteering performance is very close to the optimum end-fire performance. We conclude that optimized oversteering is a viable method for designing end-fire arrays that have better constrained directivity than broadside arrays but with a similar implementation complexity. A numerical simulation is used to perform a statistical analysis, which confirms that the maximized oversteering performance is robust against sensor mismatches
Tuning the performance of the superdirective frequency-invariant beamforming applied to end-fire arrays
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