>I wonder if it is an acoustic feature of the valley. My next
> experiment will be to get out of this valley and see if I still
> record this rumble.
>
> Any thoughts?
John,
If it is a constant frequency the odds are that it is man-made as Dan
suggests. It could be a resonsnt effect - which I have at the back of
my house when a plane flies past at one particular direction. At 45Hz
you would be looking at dimensions around 7.5metres or 25 feet. It
could be catching a roof cavity or just spaced walls. (A low frequency
grating filter)
If it is wind energised, the level would fluctuate, but a more
constant sound would indicate a mechanical source. Land topography and
wind shear could focus a distant source. Air absorption at these
frequencies is very low but could mask more recognisable engine
noises. The source could be miles away.
It sounds an interesting puzzle and recordings elsewhere in the valley
and on the edge could give clues. I live in a valley and it is
normally very quiet, but under some atmospheric conditions I can hear
village noises well over a mile away at the foot of the valley.
David
David Brinicombe
North Devon, UK
Cogito cogito ergo cogito sum - Ambrose Bierce
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