Help finding out reason for electrical phenomenon (FM radio from nowhere!)

[Syllith]

I'm an experienced electrician working at a car audio installation shop and today my salesman brought something quite interesting to my attention. We have a demo wall with many speakers, subs, amps and radios. Everything is connected via a matrix switch. There are separate switches for the radios, speakers, amps, etc. When, and only when, you have either a sub amp or a 4 channel amp selected, as well as a speaker of course, you can hear FM radio playing quietly (but not that quietly) out the speakers (mostly the rears), even when the selected radio is muted and not on the FM source. So I think the answer is obvious and one of the other radios is playing FM and it's somehow just bleeding into the audio lines. So out of curiosity I turned off every single radio and low and behold, still FM playing through. Now, I'm thoroughly confused. How is it possible that without something to cancel out the carrier wave can actual radio be playing through a speaker? Seems crazy, right? I'm sure I turned every radio off. None of them have antennas plugged in. When I turned off one of the 2 amp channels, it went away a little. Only when I turned them both off did it completely go away. If you turn the volume up on the selected radio it overpowers the FM quickly so it isn't a problem to demo.

What's going on here? I'm beyond clueless.

[helius]

It's more common to see this type of interference from AM radio, as it can be received by all manner of wires and demodulated by just a diode. If it really is FM that is more surprising, but all it really requires is a circuit that reacts with variable gain (or attenuation) dependent on frequency. The crossovers in a speaker or subwoofer are a candidate for such a circuit; although they are designed to work at audio frequencies, it's possible they have a harmonic response at RF frequencies as well. There could also be some component with a parasitic oscillation close to the FM carrier frequency, creating an unintentional downconverter.

[T3sl4co1l]

Newsflash: your radios are badly designed, and fail electromagnetic susceptibility -- somewhere in the signal path.  Complain to the manufacturer, or buy brands that don't suck.

In the mean time, ferrite beads on the various cables might help, but you can usually only get a slight reduction in interference that way.

Tim