Top Bunk, I suspect you are not familiar with the peace and quiet of the White Peak, miles from any villages or roads. It's total. To have this peace, quite unusually, drowned out by the frequent roar of usually inaudible high flying airliners IS obtrusive on anyone's radar, not least because of the rarity of its happening. But that is NOT the point of this thread.
It's not aircraft noise per se I am talking about, it's aircraft noise one doesn't usually hear, and I'd love to have conformed by someone who might know what is the physics that cause it to be heard only very occasionally.
The point of my question, which you could have a go at answering if you like, was not the obtrusiveness of the noise, but why it is such a very occasional occurrence.