Error in Finding Direction of Arrival Using a Linear Array Due to Scattering from an Elastic Cylinder

Sonar domes, with stiffeners, house acoustic transducer arrays that are used to determine the direction of arrival of nearly plane acoustic waves generated by distant sources. Scattering of the waves, by the stiffener, disturbs the pressure field and causes a difference between the actual and measured directions of arrival. The errors, when using energy and split beam correlation methods, in finding the direction of arrival (DOA), caused by the presence of a stiffener in the form of a circular elastic cylinder near a linear array of hydrophones, are investigated. Characterization of arrays of hydrophones is done in free-field conditions with no boundaries or scatterers in the neighborhood of the array. However, arrays are installed inside a sonar dome that is often constructed using thin plates and stiffeners. Acoustic plane waves from distant sources are partly reflected by the thin plates. The transmitted waves are scattered by the stiffeners and reach the hydrophones. In some cases, the frequency of interest is such that the cross-section of the stiffener is comparable to the wavelength in water and the length of the stiffener is several times the wavelength. The presence of the effectively infinite cylinder causes significant changes in the magnitudes and phases of the pressures at the locations of the hydrophones in the array (Mathew et al., 2010). Scattering from various types of cylinders is studied. Morse and Ingard (1968) present the scattered pressure, displacement, and intensity fields when a plane wave is normally incident on a rigid infinite cylinder. Junger and Feit (1972) present the pressure field when a plane wave is normally incident on rigid finite and rigid infinite cylinders. Skudrzyk (1971) presents the pressure scattered from rigid and fluid cylinders. Faran (1951) presents the calculated and measured directivity of the pressure field scattered by an infinite elastic cylinder. Akay et al. (1993) present the scattering from fluid-filled concentric elastic cylindrical shells using exact equations of elasticity and modified Donnell's shell theory. The method of separation of variables and a Fourier series expansion in the circumferential direction are used in all the above investigations.