Whilst living surviving in a quarter at Odiham in early 1980, long before we ordered any Chinooks for ourselves, the quiet of the neighbourhood one Sunday was disturbed by a distant noise (which sounded initially to me like desynchronised piston engines). This noise got louder, and louder, until eventually some 15 minutes later the sky filled with formations of C-47s, UH1,s and other rotary assets passing in a stately fashion - certainly more helicopters in the air than we had on the ground. The !st Air Cav just letting us know they were around as they proceeded up to SPTA for an exercise. The noise was .........impressive. Some weeks later Mrs S and I were in the centre of Woking when I heard that (by now) distinctive noise of a Chinook somewhere up high. Eventually a lone Chinook up at 10000 ft or so appeared - almost 35 minutes later! Whereas one approaching at 100ft was almost overhead when you first heard it.
That taught me a lot about noise propagation from the aircraft for when I also started flying them. Go high, spread the noise around, go low and you can surprise a lot of troops (but frighten a lot of animals).