Date: 09 Dec 97 03:54:26 From: email@example.com (James Matthew Weber) Organization: Customer of Access One Pty Ltd, Melbourne, Australia References: 1 2 3 4 Followups: 1
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>I was at Edwards AFB for their recent airshow, which included an >SR-71B doing a Mach 3 flyover at about 75,000 ft. The boom sounded >like a couple of gunshots from a few hundred yards. I don't know if >lower speed would reduce the intensity (I have a feeling it wouldn't) >but lower altitude and a larger aircraft would probably cancel out any >advantage of lower speed. Several points. I don't know how well sound is conductedwhen the air density is very low. SR71 flys above the tropopause, Concode flys in the Troposphere. I also suspect there are some subtle features onthe SR71 to reduce it noise signature as well as it radar signature. Even ignoring that, SR-71 and Concorde are of similar size and weight. On that basis, the 'boom' should have similar energy. However the inverse square law suggests that the boom from 75,000 feet will deliver only about 15% of the energy per unit area on the ground that the boom will deliver from 30,000 feet. Concorde usually goes Supersonic as it passes 30,000 feet, and while the aircraft approaches 60,000 feet, it only does so at the very end of cruise. It covers a lot of distance at altitudes that are a lot lower, and consequently has a large noise foot print. Concorde's other problem is the jet exhause is supersonic, even if the aircraft is not. That is a major cause of noise at lower speeds and altitudes.