An AM radio tuned to a dead band becomes an electromagnetic field detector for motors, computers, and household appliances

AM radios work by demodulating electromagnetic waves in the AM frequency band. Tuned to a ‘dead spot’ between stations, they amplify any stray electromagnetic radiation at those frequencies. Almost all electronic devices emit such radiation as a byproduct of their operation: computers, fluorescent lights, cell phones, motors, remote controls, and subway trains all generate detectable electromagnetic fields. The radio translates this invisible activity into audible sound, revealing the electromagnetic ‘weather’ in any space. An inductive telephone tap coil connected to an amplifier extends this capability to lower frequencies with stethoscope-like directional precision. Two AM radios placed close together with both tuned to a dead band produce Theremin-like interference between their internal local oscillators.

Examples

Place a cheap AM radio next to a laptop and tune to the dead band: you hear CPU activity, disk seeks, and network activity as different timbres. Move the radio near a subway door as it closes for mechanical/electromagnetic sound.

Assessment

Why does an AM radio in a ‘dead spot’ still pick up sounds near a computer? What is the difference between using a radio and using a telephone tap coil for electromagnetic listening?