So here's another Chinook question (could be applied to Lynx as well):
Why can I hear a Chinook/Lynx approaching from miles away but, once they have flown over me, the sound fades quite rapidly? In other words, I can hear a Chinook coming towards me from about 2 miles away, but can't hear it going away from me beyond about half a mile.
Obviously wind direction will have some effect but this is too regular to be just down to wind. I think it applies to all helicopters but it is far more noticeable for the distinctive sound of the Chinook and Lynx.