ASA 124th Meeting New Orleans 1992 October

2pSA2. Projection of near-field measurements of scattered pressure to the far field.

Karl B. Washburn

Earl G. Williams

Brian H. Houston

Naval Res. Lab., 4555 Overlook Ave. SW, Washington, DC 20375-5320

NRL has successfully produced the first scattered hologram of an underwater shell, ensonified by a bow-incident plane wave. The total acoustic pressure is measured as a hologram on a cylindrical surface concentric to and in the near field of the shell. The scattered pressure is constructed by subtracting a hologram of the incident field, measured without the shell, from the total field hologram, with the shell in place. Using generalized near-field acoustical holography (GENAH), the scattered hologram is processed to reconstruct the pressure and radial velocity on the surface coincident with the shell exterior. The Helmholtz integral equation (HIE) is then applied to the pressure and velocity to compute the far-field directivity pattern. The shell used in the holographic measurement had its far-field response directly measured as well: the two measurements will be compared. Sources of error, including scattered energy escaping from the ends of the unclosed cylindrical contour used in the HIE, will be examined.