ASA 130th Meeting - St. Louis, MO - 1995 Nov 27 .. Dec 01

5aPAb4. Edge wave on-axis behind a disk or aperture having a random edge.

Pinelopi Menounou

Michael R. Bailey

David T. Blackstock

Appl. Res. Labs. and Dept. of Mech. Eng., Univ. of Texas, P.O. Box 8029, Austin, TX 78713-8029

The Helmholtz--Kirchhoff intergral is used to predict the edge wave on-axis behind a disk (or an aperture) that has a ragged edge. The ragged edge is modeled as being made up of N arcs of equal angle (subtended from the center of the disk) but differing radii r[inf i]. The on-axis edge wave is thus a sum of N scattered signals, each of which has a common amplitude proportional to 1/N but a different delay time (tau)[inf i]=[radical r[inf i][sup 2]+s[sup 2][radical /c[inf 0], where s is the axial distance from the disk. A formula has been derived for the edge wave's rms pressure, in terms of N and the incident wave's rms pressure and autocorrelation function. The formula has been evaluated for incident waves that are sinusoidal, random (noise), and transient. The calculations agree reasonably well with underwater measurements [J. Acoust. Soc. Am. 92, 2359(A) (1992)] made with a spark source and various apertures and disks (biradial, triradial, and ragged). When N is large and the range of values of (tau)[inf i] is large enough, the rms value of the edge wave approaches zero. [Work supported by ONR, NASA, and the ARL:UT IR&D program.]