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Comment by thyrsus | Hacker News Reader

Does someone know it's usual frequency? When it's quiet in my house there's a hum at around 50hz, but its very constant - no 13 to 300 seconds, it's always there. I first assumed it was someone in the neighborhood running a motor, or maybe the water plant a couple miles away, but when it was particularly noticeable during a widespread blackout here (in part, due to the quiet - and eliminating the likelihood of an electric power effect), my next ideas were a resonance from traffic on the interstate a couple miles away, or maybe someone even further doing some drilling (there's been discussion of fracking starting soon a dozen miles away or so, but it's not supposed to be happening yet, and I first noticed several years ago).

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Keep in mind the black out wouldn't rule out any electric device that had batteries or capacitors as part of it. I had a smoke detector that made a slight humming, took me forever to figure out where it was coming from, and replace that smoke detector.

Battery-powered smoke detectors will likely be running on DC so coil hum seems unlikely.

Unless it has a switching converter or regulator. Even when running at high frequencies they'll often generate harmonics in the audible range. It can take a heck of a lot of effort to keep a circuit quiet and nonmicrophonic.

Such as land line phones which still have power even in a blackout it's separate from power lines.

The phone line power must be AC I can't see such long runs being DC.

Actually, it's 48 volts DC (http://en.wikibooks.org/wiki/Communication_Systems/Telephone...). A few years ago I got to tour a central office for a small telephone/cable company in a suburb of Houston and the phone lines were actually powered directly by massive banks of lead-acid batteries which were then constantly kept floating by the mains supply.

Also there is radio waves interacting with passive objects.

I have been working with radio for the last few years and while my theory isn't that strong a colleague of mine was pretty clear that AM signals from planes (or control towers) had been observed to make detectable sound levels in nearby houses.

50 Hz is very characteristic for a power line close by but as you already ruled that out I have no clue. Nonetheless much to high for any seismic noise.

He's in the US, which rules out 50Hz at any precision. He needs to take measurements.

On the other hand, without measurements, 60 Hz may be close enough to 50 Hz to include it in "around 50 Hz", depending on the OP's ability to estimate sound frequencies.

But yes, measurements are the way to go.

On a piano, it's closest to the C three octaves below middle C, so around 33Hz. I regret my lack of perfect pitch.

I hear that too, mainly at night, a constant droning that seems to come from very far away outside and from no specific direction, but bothersome enough to keep me awake.

Not sure how it would work during a blackout, but it sounds like you're listening to a mains hum. http://en.wikipedia.org/wiki/Mains_hum

Depending on the blackout cause there could still be live powerlines nearby. If a local transformer blew then the lines feeding it would likely still be live until they shut them off to replace the transformer.

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