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.

On some nights, REACH Hercules flights crawling-out westbounds sound like
someone moaning in the garden outside...

The tone and volume of the audible sound varies considerably depending
upon cloud cover. However, it is simple and practical to sit on a
park bench on a heavily-overcast day and count the aircraft passing
overhead, high above the cloud deck, and to associate each aircraft
with traffic heard on the Scottish Upper frequency. On a clear
day, a pair of binoculars will confirm the livery, to a degree
( ``ah, quad with four red nacelles and a reddish tail; must be
VIRGIN 75...'' )

BTW, I'm in Belfast.

Thanks

Andrew

--
Andrew Bunting