> 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.
Can you please explain "step function"? Is that like a square wave?
> 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.
I'm going to have a play with "fixing" them, but I might end up just leavin=
g them in as Keith Smith has suggested. Did you apply your filter to the wh=
ole recording, or just to those sections? I'm wondering if simply hand edit=
ing the wave form would work. I haven't tried that before.
> 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
You could be right, and I have no idea what military operations there might=
be that could create these noises, especially if it could be coming from a=
very long way away, as others have also suggested. But I've now found at l=
east 90 booms, at random intervals of the 90 minutes I've checked so far. T=
hat would have to be a very active aircraft, wouldn't it? It makes me think=
a gun of some kind is more likely, but anything is possible.
> 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.
That could easily be a windshield problem. As well as a layer of lycra stre=
tched over the array, I have a badly constructed wire frame around the whol=
e thing supporting a second layer of hessian. I'll have to see if I can see=
these peaks you refer to and see if I can reproduce and then reduce it.
> Does any of this make sense? :-)
Mostly, although I'll have to take your word for things like "stimulated a =
> BTW, a beautiful recording. :-)
Thanks, very much appreciated, and to the others who liked it too. It's mor=
e important to me that the rig is working ok than that I find out what thes=
e noises are. I'll try to put some more on Soundcloud soon.