Edge wave on axis behind an aperture or disk having a ragged edge

Diffraction by a circular aperture or disk having a ragged edge is investig ated. Theory and measurements are reported. The ragged edge is modeled as N arcs, of differing radii a(i), each of which contributes a scattered signa l to the edge wave on axis behind the aperture or disk. The amplitude of ea ch scattered signal is proportional to the angle of the are, and the corres ponding time delay is root a(i)(2)+s(0)(2)/c(0), where s(0) is the axial di stance from the aperture plane and c(0) is the sound speed. Kirchhoff theor y is used to make the calculation. A formula is derived for the rms pressur e of the edge wave in terms of the rms pressure and autocorrelation functio n of the incident wave. The formula can be evaluated for incident waves tha t are sinusoidal, random (e.g., noise), or transient. Predictions agree rea sonably well with underwater measurements made with a spark-generated pulse incident on various apertures, The main result is that making the edge rag ged reduces the rms pressure of the edge wave. Indeed, an edge profile is p resented that, for a given frequency and axial observation point, eliminate s the edge wave completely. (C) 2000 Acoustical Society of America. [S0001- 4966(99)02311-5].