Computer tech expert William Entriken has created a C library that can make a computer emit radio waves even if the device doesn't include any radio transmission hardware.

His program, called System Bus Radio, is written in C and relies on code that makes the computer's CPU emit electromagnetic radiation at certain frequencies.

The radiation is strong enough to escape the computer's shielding, and with the proper timing at which data is processed in the CPU, it can be forged into radio waves that carry off information away from the target's computer.

During tests, Mr. Entriken says he managed to transmit a version of the "Mary Had a Little Lamb" song over a distance of up to two meters and one meter if a drywall separated the computer and the receiving antenna.

You can get System Bus Radio from GitHub, test it on your own hardware, and add your results to Mr. Entriken's findings.

Worth reminding that this escape method was invented by a human-level intelligence. A Bayesian superintelligence may be able to devise much more cunning methods.

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This also works with magnetic field frequencies, which go through faraday cages, and allegedly any hacked CPU can generate them by overloading certain parts at certain frequencies. 

I'm not sure whether the average smartphone's magnetometers can pick them up (the average smartphone can definitely emit them). But high-priority military computers need to be surrounded by massive iron blocks ("ferromagnetic shielding") in order for an air gap smaller than 100 feet to actually be an air gap.