A new technique of acoustic mode filtering in shallow sea

A new technique of normal mode filtering in an isovelocitcyh anneal singe n equispaced line Array is presented. Filtering is achieved by steering nulls in the directions of arrival of the Quasi plane wave pairs associated with the normal modest hat are to be rejected .A theoretical Analysis of the problem is presented for both horizontal and non-horizontal arrays. The Minimum number of array elements and their weighting coefficients e determined for each Case. The null-steering technique offers several advantages over other techniques of mode filtering. The weighting coefficients are independent of the array depth. A horizontal or vertical array of 2M - 1 elements with real weighting coefficients or a horizontal array of M elements with complex weighting coefficient is sufficient to select one mode from a set of M modes. There is no constraint on imperilment spacing. The filter performance is equally good under all types of bottom conditions. If a horizontal array is deployed this method of mode filtering can be used in a horizontally stratified channel also. The null-steering method gives perfect filtering if the orientation of the array and the modal wave numbers are known exactly. An analysis of the sensitivity of the mode filter to errors in the assumed values of these parameters is also presented.