Multibeam Sonar Theory of Operation

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Introduction to Multibeam Sonar: Projector and Hydrophone Systems Multibeam Sonar Theory of Operation Figure Chapter 3 - -7: Three-Dimensional Beam Pattern for Two Projectors with Spacing l/2 One useful way of interpreting Figure Chapter 3 - -6 is as a plot of the angles at which the energy of an acoustic pulse is directed. In the figure, the highest level of acoustic energy is directed perpendicular to the axis of projector separation. Objects in these directions are ensonified with the most energy and return the strongest echoes. Objects in other directions return significantly weaker echoes. This selective projection of energy is called directivity. The beam pattern provides a measure of the directivity of a system— projector arrays that direct the bulk of their energy at a very narrow range of angles are said to have a high directivity. Real projector arrays generally have more than two projector elements and have complex beam patterns. One common array configuration is a simple extension of the two projector array— an arrangement of many projectors in a straight line called a line array. The beam pattern of a multiple-element line array is pictured in Figure Chapter 3 - -8. Although detailed mathematics are required to compute this pattern, it is essentially just an extension of the reasoning used to find the two-projector array pattern. This complex beam pattern can be used to point out the features of all such patterns. Page 3-8 Copyright © 2000 L-3 Communications SeaBeam Instruments No portion of this document may be reproduced without the expressed written permission of L-3 Communications SeaBeam Instruments
Introduction to Multibeam Sonar: Multibeam Sonar Theory of Operation Projector and Hydrophone Systems Figure Chapter 3 - -8: Beam Pattern of a Multiple-Element Line Array The bulk of the energy in the line array beam pattern is in what is called the main lobe. The direction of the peak energy projection— the center of the main lobe— is called the maximum response axis (MRA) of the beam pattern. The width or beam solid angle of the main lobe, which is a measure of the pattern's directivity, is twice the angle from the axis to the half power point on the pattern— the angle at which the projected power is exactly half that of the axis. This is referred to as the beam width measured to the -3 dB point, where the projected power is -3 dB that of the axis (these points are almost equal, as -3 dB corresponds roughly to 1/2). The main lobe of the line array pattern is narrower than that of the two-projector array pattern. In general, the larger an array or projector system is in a dimension, the narrower the main lobe of the beam it projects in perpendicular directions will be. A good first approximation for the width of the main lobe of a pattern of a system with size D (which can be the physical size of the transducer in a single-projector system, or (N - 1) × d, the total length of N elements in an array with spacing d) transmitting at a wavelength λ is: half power beam width (degrees) = 50.6 × λ/D (3.6) Copyright © 2000 L-3 Communications SeaBeam Instruments Page 3-9 No portion of this document may be reproduced without the expressed written permission of L-3 Communications SeaBeam Instruments
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Multibeam Sonar Theory of Operation

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