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Re: Background thumps in recording

Subject: Re: Background thumps in recording
From: "Keith Smith" keithahsmith
Date: Mon Nov 5, 2012 8:50 pm ((PST))
Ah, me thinks RAAF Edinburgh 150+mi W is a far more likely candidate.


On Mon, Nov 5, 2012 at 7:01 PM, Keith Smith <> wrote:

> 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 130m=
i
> 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!
> k
>
>
>
> 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
>>
>>
>>
>
>
>
>
>


--
Keith Smith

Keith Smith Trio, Northern Lights =96 Altai Khangai - www.keithsmith.ca
Photography - www.mymountains.ca












"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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