commgrp@silver.bacs.indiana.edu (01/11/90)
To: sci.military and sci.electronics I read somewhere that the Viet Cong had a "low tech" method of detecting incoming B-52 stikes: They dug bell-shaped holes in the ground, which were resonant at very low audio frequencies. The detector was a man sitting in the bottom of the hole, listening for the rumble of high-flying jets. Infrasonic detectors (possibly consisting of long pipes or buried air- chambers) have detected shock waves from nuclear air-blasts more than 1000 miles distant. The U.S. Weather Bureau (or whatever its present incarnation is called) has several arrays of infrasonic "microphones" which detect long-period atmospheric waves generated by the jet stream, aurora borealis, and other effects. I'm looking for references (especially construction details) for the above and other methods of detecting atmospheric-pressure waves having periods greater than 1 second. The information is for a friend who is studying several mechanisms of air movement in natural caves. The electronic altimeter circuit in _Modern Electronics_, Jan. 1990 p.42 looks promising. [Certain caves which have enormous volume but only one small entrance can be extremely sensitive detectors of air-pressure changes. A famous one is in Wind Cave National Park in South Dakota. Wind Cave is like a giant aircraft vertical-speed indicator; air roars into or out of the entrance in response to barometric changes. 50+ miles of cave have been mapped, but known passage accounts for only 2% of volume calculated from air-flow measurements.] The British used steerable arrays of horns and microphones to listen for Nazi planes approaching the coast. Were these effective, or were they a ruse to cover radar, like the carrots/vitamin-A story recently discussed in sci.military? -- Frank Reid W9MKV NSS 9086F reid@gold.bacs.indiana.edu P.O. Box 5283 Bloomington, Indiana 47407 USA