My last deployment our FOB was visited frequently by ANA and UN Mi-8/17s (plus assorted other flavors of US, ISAF, and contractor helicopters). There was masking terrain about 6-8 km out from the flightline side of the FOB and once incoming aircraft cleared that ridge line you could hear them the rest of the way in, unless there was background noise drowning it out. That's the only time I can think of off the top of my head where I dealt with hearing aircraft coming where I had a known distance terrain feature to index on.
Not sure on a solid figure for how far out you could pick them up if those ridge lines weren't in the equation. Besides masking aircraft on the far side, I'm not sure if the ridges didn't help hear aircraft on the near side by reflecting sound waves back towards the FOB.
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