> I made multiple recordings in various rooms of the house and the
> cellar and porch today, and outside pointing the microphone in four
> directions. The noise is there in all recordings, loudest outside,
> quietest in the cellar.
>
> The frequency is low, around 45 Hz with clear harmonics. The
> intensity is low, around 5 dB SPL inside and about 15 dB SPL outside. =
> Those numbers are very, very approximate. I do not have a SPL meter
> that reads that low, so I extrapolated based on the levels in the wav =
> file. Also, in order to hear it at all and have the harmonics show up =
> in a spectrum image, I had to apply a High Cut filter above 100Hz.
> Otherwise it is buried in mic noise and whatever else.
>
> It's not a problem, I'm just curious what it is in the environment
> that is producing this sound. Is this normal background noise? We
> live in a small valley between two modest ridges. The sound seems to
> be louder by a few dB when the microphone is pointing south, down the =
> valley. 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.
I would suspect that someone has a generator running. Low frequencies trave=
l a long way.
-Dan
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