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Comment by genericuser | Hacker News Reader

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.

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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.

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.

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.

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