Circuit Sniffing: Eavesdropping on Hidden Magnetic Music
Nicolas Collins · 2009
Radios make the inaudible audible. Unlike microphones and amplifiers, which merely make very quiet acoustic sounds much louder, radios pick up electromagnetic waves that have no acoustic presence whatsoever and translate them into signals that can be heard through a loudspeaker. Radios are manufactured for listening to intentionally transmitted electromagnetic waves (i.e. those sent from radio stations) from which they extract music and speech through a process of demodulation-multiple stages of amplification, filtering, and frequency shifting. But radios can also be used to sniff out other types of electromagnetic signals, such as those emitted by lightning, sunspots, Aurora Borealis, meteorites, subway trains, and a gaggle of household appliances (see Art & Music 1 “Mortal Coils”). Generally speaking, AM radios (the cheaper the better) do a better job of picking up these “spurious” noises than FM radios. (In fact, the invention of FM technology was celebrated as the triumph of signal over noise, thanks to its superior rejection of exactly the kind of weird stuff we want to hear here.) Put batteries in the radio and turn it on; if it has a band select switch set it initially to AM. Tune it to a dead spot. Try moving it around various electrical appliances: fluorescent lights, electric motors, computers, portable CD players, cell phones, MP3 players, infrared remote controls, and controllers for RC planes and cars are especially noisy. Fire off a camera flash next to it. Experiment with tuning the radio to different stations, in-between stations, and to the dead bands at either end of the dial.