| ▲ | jetrink 2 hours ago |
| At first I thought the "unmanned tunnels" description was just a way to avoid broadcast regulator scrutiny, but it does look like it's genuinely designed to be used underground as part of an emergency alert system. That led me to "leaky feeders", a type of broadcast antenna used in mines and tunnels. https://en.wikipedia.org/wiki/Leaky_feeder |
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| ▲ | jasonjayr an hour ago | parent | next [-] |
| I'm curious about challenges (what's bad with AM broadcast in an unmanned tunnel?) and why the formally verified killswitch was necessary? |
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| ▲ | meindnoch an hour ago | parent [-] | | A tunnel basically acts as a waveguide. A sufficiently shaped AM transmission can turn the tunnel into a cavity resonator, basically a huge microwave oven. There have been multiple cases where people were cooked alive in mines and utility tunnels, due to faulty AM equipment broadcasting at the wrong resonant frequency. | | |
| ▲ | samschooler 7 minutes ago | parent | next [-] | | I did a more aggressive internet search. This seems not possible given physics, as well as not documented (at least in the US) in the CDC Mine Accidents Database [0], which has been recording mine accidents since before the discovery + invention of AM radio. [0]: https://wwwn.cdc.gov/NIOSH-Mining/MMWC/MineDisasters/Table | |
| ▲ | albumen an hour ago | parent | prev | next [-] | | Fascinating. Any references? A cursory web search reveals nothing. | | |
| ▲ | kurthr 3 minutes ago | parent | next [-] | | This is basically hilarious. Leaky Feeders are a few watts. and even a high powered multi-kW AM radio with 200m wavelength wouldn't do much in a rough walled tunnel multiple sq meters in cross section. It's both too large for there to be significant power density, and much less than a wavelength in diameter except in length where the tunnel passively attenuates the signal. | |
| ▲ | andrewstuart 8 minutes ago | parent | prev [-] | | Fascinatingly false. |
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| ▲ | CamperBob2 42 minutes ago | parent | prev [-] | | That is hilarious. You win the Internet for February 18, 2006. |
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| ▲ | _moof 2 hours ago | parent | prev | next [-] |
| I've also seen these used to add audio to art installations in commuter tunnels. |
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| ▲ | cbdevidal 2 hours ago | parent | prev [-] |
| Thank you, I too was confused at the purpose of this |
