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Comment by ronreiter | Hacker News Reader

Wow, can anyone explain this?

The voltage applied to the cable is directly proportional to the audio signal, overlaid on a ~1 MHz carrier. All you need is a mechanism that transforms those voltages into vibrations or pressure waves. Presumably, since it seems to happen only when he's striking an arc, it's related to the local heating of the air in the gap ... But I guess it could be electromagnetically induced vibrations of the cable or the tower structure ...

There used to be stories, not entirely urban legend, of people who lived close to AM towers hearing the station in the wiring of their home, or in their dental appliances. Not impossible, considering that many AM stations broadcast with 50kW of power (like the one in this video) and can be received across a large swath of the country.

There used to be stories, not entirely urban legend, of people who lived close to AM towers hearing the station in the wiring of their home, or in their dental appliances.

Reminds me of a Gilligan's Island episode, Hi Fi Gilligan: https://www.youtube.com/watch?v=3x1XHv0sxro

Lucille Ball talking about picking up radio signals in her fillings. https://vimeo.com/72473069

That was one specific station - the infamous WLW / W8XO - the first and only US station authorized to operate at 500kW of power.[1]

To give you an idea of how powerful that is, take a look at their coverage map: http://i.imgur.com/Aigz1Zh.jpg

The back story is kind of interesting. It was run by Powel Crosley - a sort of 1930's version of Elon Musk. Advertising revenues from the station were so successful he applied to increase the limit from 500kW to 750kW(!).

It's a big deal in the ham community, but I only learnt latter that while US stations were limited to 50kW - other parts of the world didn't have such restrictions. Parts of the Middle East have 2,000kW stations. [2]

You can get a full download on the Crosley WLW station in this video: https://www.youtube.com/watch?v=CbHjcwIoTiY

[1] http://www.ominous-valve.com/wlw.html

[2] https://en.wikipedia.org/wiki/Transmitter_Solt

Actually, Solt is not Middle East, but in Central Europe. (I live ~50 km from it.) But yes, it's 2 MW, and at night, it can be heard using conventional receivers as far as Belgium.

Corrected link: http://www.engineeringradio.us/blog/2012/12/the-continental-...

Well, the Warsaw Radio Tower used to be the tallest man-made structure in the world until it collapsed due to a guy rope failure - and it had a 2 megawatt transmitter, it could be received literally anywhere in the world:

https://en.wikipedia.org/wiki/Warsaw_radio_mast

I grew up about 5 miles from that antenna. I can't say I ever heard the radio through the wiring in my house. The Voice of America antenna array down the street though? I could hear it clearly through my stereo speakers when everything was turned off.

It's been dismantled, but I can imagine the number of ghost stories that it spawned. Just sitting in your house, and then you faintly hear a man speaking German...

It's weird seeing people on reddit who grew up in the same town as you, but it's even weirder seeing it on Hacker News for some reason.

I never heard the broadcasts through metal objects in the house either, but maybe they scaled back the power in the time I grew up there. Never heard any VoA broadcasts either.

It's definitely not an urban legend - I witnessed this first hand at a high school friend's house, where headphones attached through long extension cables across the room would pick up the nearby AM transmitter.

I have a friend who lives as close as one can legally live to a 100kW FM transmitter. He received a pair of very cheap speakers when he built a new computer but was unable to use them because if they were plugged in the radio signal would come right out of the speakers.

It seemed that the signal path into the speakers was through the input wire (rather than, say, the power cable or the wires between the amplifier and the cone). From what I understand from my undergraduate analog signals course I'm suspicious of the signal actually coming from the transmission tower itself, unless the speakers had just the right configuration to act as a simple diode-rectifier envelope receiver (terminology likely to be incorrect), which I would think of as unlikely for an FM signal.

It may be that the signal came from some other path related to the relaying of the signal from the studio (~1 mi away) to the tower, but I don't know enough about the specifics of the radio station's configuration to say more.

That happened to me sometimes, living in the UK --- my computer speakers would come on in the middle of the night and start whispering to me in French. It was exactly as creepy as it sounds.

I think I did eventually pin down which station they were picking up, but I've forgotten now. Interestingly, since I've moved to Switzerland, it's stopped. Maybe the signals don't get through Swiss customs.

A diode isn't actually required for AM reception, it just happens to be one of the better ways if doing it. In fact, any non-linearity in a circuit will act to demodulate AM signals, as the video and your friends experience nicely demonstrate.

He was talking about FM, not AM. Which makes it rather weird, as you actually need more of a circuit to decode it.

In college I lived in an apartment near a highway and my keyboard amplifier would pick up CB radio from truckers. I had to physically unplug it at night time.

I had a pair of really cheap computer speakers once that would catch faint FM signal whenever they were powered. I was in a 6th story loft at the top of a hill with a clear view for maybe 30km from my window but not actually close to any transmitter. I blame the 5$ speakers, not the FM transmitter.

That said, I didn't live 300m from 100kw...

Same way as a plasma speaker

https://www.youtube.com/watch?v=PJpn2PzhvP4

Or, on a larger scale, a Zeusaphone/Thoremin: https://www.youtube.com/watch?v=3ff_AXVlo9U

https://www.youtube.com/watch?v=hgll-XTqcS4 (youtube-dl with mpv fullscreen recommended)

Dumb question is it also creating the piano like sound in that video with the electricity? Very cool to watch.

Yes! That's all done with the plasma channel expanding and (therefore) contracting at audible frequencies.

(edit: I'm way wrong on the piano. arjie's correct about the backing track)

There's a backing track.

It would be cool to see a guitar amplifier that worked like this. It seems dangerous, but that would be awesome to hear/watch.

I guess it is because of the use of amplitude modulation [1], where the envelope ("strength") of the carrier (that is some higher frequency signal) follows the signal that is modulated onto it (speech, in this video). With say FM it wouldn't work, I presume.

[1]: https://en.wikipedia.org/wiki/Amplitude_modulation

If you listen to an FM signal interpreted as AM you can often still make out the audio, it'll just be very clipped and noisy. That was the result when I did this with an RTLSDR, at least

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