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By A. Waite

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2 What is the source level, SL, of a projector radiating 40 kW of acoustic power and with a directivity index, DIt, of 15 dB? 3 What is the safe maximum radiated power to avoid cavitation, given a 100 ms pulse of frequency 20 kHz and a planar array of 2 m length by 1 m height, containing circular elements closely packed and operating at a depth of 10 m? 4 The total intensity of the sound in an octave from 2000 to 4000 Hz is 80 dB relative to the intensity resulting from a sound pressure of 1 pPa.

In an active system it is the combined, transmit and receive, beam pattern which is important and it is hardly ever desirable to hrther reduce the sidelobes at the expense of SL - far better to adequately shade the receiver elements only. 8. 6). 9). 30. 3. 4. 5 dB. Note that all of the other weighting functions broaden the main lobe more than the Dolph-Chebyshev -30 dB weighting function. The theoretical sidelobe levels are generally lower but they are seldom fully achieved in practice. Element failures and, in the case of a towed line, curvature of the array, ensure that it is unsafe to expect sidelobes much better than, say, 25 dB down on the main lobe.

Note that the 20 dB beamwidth is about double the 3 dB beamwidth. 1 What are the 3 dB, 10 dB and 20 dB beamwidths for a line array of length 6;1? What are the bearings of the first-, second- and third-order sidelobes relative to the direction of maximum intensity? 9 Shading Except for the simple dipole with element spacings less than or equal to 1/2, all the above beam patterns have significant sidelobe levels. 5, 18 and 21 dB below the peak level of the main lobe. Strong signals will be detected through the sidelobes of adjacent beams as well as, correctly, in the main lobe of the beam at the bearing of the signal.

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Sonar For Practicing Engineers by A. Waite

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