ASA 127th Meeting M.I.T. 1994 June 6-10

2aPA11. Sonic booms near and beyond the edges of primary carpets.

Allan D. Pierce

Dept. of Aerosp. and Mech. Eng., Boston Univ., 110 Cummington St., Boston, MA 02215

Sonic booms received on the ground tend to be restricted to a region of finite lateral extent below the flight track. This occurs because of refraction and because the effective speed of sound, even with winds taken into account, decreases with altitude in the lower atmosphere. Not all rays proceeding initially downwards from the flight track within an allowable range of initial directions will reach the ground. The restricted region, which can be reached by rays impacting the ground, is known as the primary carpet. However, weak rubles are heard in the nominal shadow zone beyond the edge of this carpet. A full wave theory is necessary for explaining waveforms in that region, and the present paper gives a matched asymptotic expansion technique for a suitable approximate full wave theory that involves a relatively small number of parameters. The outer solution is derived from the structure of the system of rays, the impact near the corridor edge, the inner solution involves a solution of the parabolic equation and results in the special functions encountered in the diffraction of sound over the tops of hills. [Work supported by USAF, Armstrong Laboratory, WPAFB.]