Yesterday morning I made a 20 minute recording at the local wetlands with m=
y new SASS array (4 x EM172's in a Rob Danielson style wooden structure, ha=
lf finished - no nose baffle or internal baffling). I also made a recording=
using the Sony PCM M10's internal mics for comparison.
When I loaded into Audacity and displayed the spectrogram, I see lots of ba=
nds on the right channel, about 400Hz apart, to 20kHz and beyond. I can't =
see any sign of them in the left channel, and I can't believe this thing ha=
s such good separation that it could be environmental. I can't hear anythin=
g odd, but with my hearing, that doesn't mean much.
I also see no sign of it in the recording using the internal mics.
So it must be generated in the mic circuitry? I just made another test reco=
rding with it on the table here beside me, and I see no sign of the bands o=
n that recording either.
So maybe it picked up some interference with it's unshielded wiring? Maybe,=
but would that look like this? And I was at least a kilometre from the nea=
rest powerlines. The site is an old saltworks, so there may have been some =
pumps running a bit closer than that. The lead to the M10 is only metre lo=
ng, and shielded, but there are a few inches of unshielded wiring in the ar=
ray.
More intriguingly, the first 20 seconds of the recording don't show the ban=
ds. This includes the sounds of me wrapping the array in a towel for wind p=
rotection. The last part, where I unwrap it again, also doesn't show the ba=
nds. That I don't understand at all. Unless perhaps my close presence someh=
ow damped some interference?
I've uploaded one minute of it in wav format if anyone wants to have a look=
at it: http://soundcloud.com/petershute/sass-test-harmonics. The first 20 =
seconds is very noisy and clipped, but I've had to include it to show the b=
and free section.
Peter Shute
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