I accidentally recorded radio. How?

Question

I plugged in my mic and recorded with a program called "Audacity". When I played the recording back i had accidentally recorded radio channel AM 790 which, at the time was broadcasting the Michael Savage show. I tuned in and the record wave moved as he spoke. I was wondering how this is possible. Is there something that can act as a receiver in the microphone? (microphone is a generic white table mic that disconnects from it's base.)

Answer 1

Well, it happened, so obviously yes there is circuitry acting as a receiver. Happens all the time. We've gotten reports of people receiving radio signals in their teeth - but more often speakers acting as receivers. So a mic isn't that much of a stretch.

-a guy named duh

Answer 2

Any unshielded wire, such as your microphone cable, will act as an antenna to some extent. The changing electromagnetic field created by a radio signal causes current to move in the wire.

When an AM radio signal is strong enough, it can get accidentally demodulated and produce understandable speech. All it really takes to demodulate AM is something in the circuit that rectifies the waveform, i.e. only allows current to go in one direction and blocks current in the other direction. Altering the electrical signal like that recovers the audio waveform that was used to modulate the radio signal. It's fairly common for a circuit to have something like that inside, even if it wasn't meant to. You can also see that effect in speakers.

The reason it doesn't work as well as a regular AM receiver (you're only receiving from one station) is that it doesn't have the tuning filters and amplifiers that would be used when actually trying to receive a radio signal. If you keep having this problem, get some clamp-on ferrite beads for your microphone cable. The ferrite beads add more inductance to the cable, which weakens the higher-frequency signals, so they won't be demodulated accidentally.