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Old 20th October 2017, 01:39 PM   #155
Checkmite's Avatar
Join Date: Jun 2002
Location: Gulf Coast
Posts: 27,048
Originally Posted by Alphaba View Post
OK, I did exactly that, to focus, in focusing on a shorter, more accurately representative sample of 2.31 seconds of duration:

Then in magnifying the "size" [Taille] of the frequency spectrum from 1024 to 65536 [changing Audacity's language from French to English on the fly had a funny effect, which disappeared after a restart], which gave the following spectrum [screencapped]:

Lots of well defined peaks and harmonics in what appeared as an undifferentiated mound at low "sizes". Both 40Hz and 50Hz AC peaks and their harmonics (until ~240Hz) are present. And also 15, 17, 25, and 30Hz peaks (all those ± 2Hz).

The mound morphed into a fancy fractal.
Wherever all the "infrasound" and "ghost" absurdity came from I don't know; I wasn't aware that anyone anywhere was proposing that. But as for this noise - which is obviously a quite real thing - it really sounds to me like it does have a similar character to other noises produced by devices that are designed intentionally to irritate; i.e for crowd control, or by commercial ships to repel attackers and protesters, that kind of thing.

Let's assume for the moment that it's not. What might it be then instead?

With respect to dann, it doesn't sound like any modem I've ever personally heard or have been able to find a sample of online. I suppose it's possible it might be some other kind of digital data mode. I'm familiar with some of these and have been trying to match it, but it just doesn't sound quite like any I personally know about - closest it comes is maybe PSK31 - but it doesn't seem to contain any of the "inflection" that common digital modes have. Plus you run into the problem that you actually need an HF receiver to pick them up and turn them into audible sound; it's not like someone would be "accidentally" physically hearing a PSK31 or RTTY signal because of a chance building arrangement or a badly made speaker component picking it up and resonating.
--- Carlos S., 2002
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