Hi David,
Thank you for your response. I see I wrote kHz where I meant Hz -oops!
I notice the FlightAware website provides altitude information for flights, and
from that it looks like most jet flights over Oregon for trips longer than a
few hundred miles are cruising above 30 thousand feet (or ~10 kilometers).
Your description of the audibility limit makes sense to me, from what I have
observed.
For most uses, I reject any material with such noise intrusions - especially if
audible anywhere between 100 Hz and 10 kHz.
John Hartog
--- In "Avocet" <> wrote:
>
> > I have not yet looked into the actual altitudes, but considering I
> > usually record at least 200 miles from the nearest major airport,
> > these flyovers must be 10 thousand feet or higher(just a guess),
>
> John,
>
> Jets are at a cruising height over me at about 35,000ft or about 10
> kilometers. I've got a set of graphs of air absorption at various
> frequencies and it comes out as around -10dbs below 200Hz but -70 dBs
> down at 1KHz. That's in addition to the drop due to distance. The
> distance drop from 100 metres to 10km is another 40dB, say 100
> dBA -40dB -70dB at 1KHz. The audibility limit happens somewhere in
> between 200 Hz and 1KHz.
>
> That's the theory - the practice is that the jet noise is well down at
> bird tweeting frequencies and cars a mile away are more of a problem
> here.
>
> I'm fortunate to be mostly under transatlantic flights, but as I said
> height is crucial to jet noise.
>
> Concordes used to go subsonic over the Bristol Channel but
> occasionally caused a sonic boom which would spook all the pheasants
> in the area in unison. :-)
>
> David
>
> David Brinicombe
> North Devon, UK
> Cogito cogito ergo cogito sum - Ambrose Bierce
>
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