> But in one recording, there is a thumping noise at random intervals,
> right through the two hour recording, which starts just before
> sunrise. Could I please have some opinions on what these noises
> might be?
Peter,
My initial thought was that this was caused by a pressure wave, in
other words a low frequency step function which you can get from a
sonic boom.
A theory is that this stimulated a local resonance as there were peaks
in the boom at 24Hz and at roughly 11Hz intervals above that up to
around 100Hz. I constructed a brutal HPF with a 70dB per octave step
between 100Hz and 200Hz and it "cleaned up" the booms except for a
slight "ruff" at the first boom, which might have been
intermodulatuion as the level was high. The other booms became
inaudible but the track sounded a bit thin of course.
There was also a "pre-boom" 1 second before the first loud boom which
could be explained by a supersonic origin as you sometimes get with a
sonic boom or a distant explosion. When Concorde was flying, we used
to hear this double boom in SW England from far over the Bristol
Channel before it went subsonic. I haven't looked up the air
absorption factors but a strong shock wave could be some tens of miles
away to have a LPF effect from HF air absorption as shown up by the
power spectrums.
In the parts of the recording remote from the booms, the power
spectrum shows a number of peaks 4Hz apart. This is equivalent to an
echo 42 metres away which is unlikely. Something, possibly a
windshield resonance is colouring the recording, and this would not be
a problem except when it is "rung" by a pressure wave. A "car door
sound" is a similar resonance excited by a slam impulse.
Does any of this make sense? :-)
BTW, a beautiful recording. :-)
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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