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Re: Background thumps in recording

Subject: Re: Background thumps in recording
From: "Keith Smith" keithahsmith
Date: Mon Nov 5, 2012 6:01 pm ((PST))
David! There you are! I was hoping you'd show up.
I have some work to do, to see the resolution you're talking about -back to
the manual I guess.

I was looking for 'gunnery ranges' around Murray Sunset. A GE search
brought up Port Pirie Aerodrome, which appears to be military, about 130mi
NW. Might that be a source of sonic booms?

I'm not hearing or seeing the 'pre-boom'. Is it around 22s?

Does it make sense? Think so. I need to absorb it all.

Yes indeed, I think this must make the list of very good sound recordings.
Congratulations, Peter!

On Mon, Nov 5, 2012 at 6:28 PM, Avocet <> wrote:

> **
> > 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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