In message , Andrew Bunting
writes
gary pearson wrote:
I don't know if they're audible to the human ear at that height, but you
probably couldn't hear them over normal background noise anyway.
Not a chance at FL 400 and very unlikely even at FL250.

I live under the approaches to the MIMKU / GOMUP / NIBOG Atlantic entry
points and I can assure you that modern ( Stage III ) traffic is easily
audible from outside as it passes overhead, even up to FL370. There is
little commercial traffic above that level but biz jets are just about
audible at FL410.

Has anyone noticed the phenomenon which I have named 'the acoustic
horizon'? I live about 5 miles from the end of the runways at
Manchester. I've noticed that I can see aircraft climbing towards me but
cannot hear them. Then, suddenly, the engine noise is audible and
rapidly increases. There is no slow fade in to audibility.

I've come to the conclusion that the change in atmospheric pressure with
height causes the sound to bend downwards in front of the aircraft so
that nothing is heard until this sound wave sweeps over my location.

Mike
--
M.J.Powell