Interesting analysis David. What affect would diffraction through trees in a
forest have on frequency of an object passing overhead?
John Hartog
rockscallop.org
--- In "Avocet" <> wrote:
>
> > Just take a listen what I recorded this spring out on remote public
> > lands in Central Oregon the middle of the night. I believe this is
> > anthronoise from an aerial drone - not natural sound.
> > http://snd.sc/U9BV3X
>
> John,
>
> An interesting recording. On a power spectrum there is clear banding
> which lowers in frequency and then rises again as it passes,
> suggesting a comb filtering between direct sound and a reflection. At
> the low point, the bands are 120Hz apart giving an effective path
> length difference of about 2.8metres from a reflector about 1.4met
> away at the highest passing angle.
>
> The engine/prop noise drops from 1514 to 1321 giving an airspeed (not
> allowing for angles) of roughly 50m/s or 110mph, or in practice,
> greater than that. On the spectrum, as the engine/prop whine passes
> through the comb filter bands, it cancels and reinforces as would be
> expected.
>
> These are rough figures but probably consistent with a prop powered
> aircraft passing not directly overhead.
>
> David
>
> David Brinicombe
> North Devon, UK
> Cogito cogito ergo cogito sum - Ambrose Bierce
>
"While a picture is worth a thousand words, a
sound is worth a thousand pictures." R. Murray Schafer via Bernie Krause.
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