Here’s the situation: I have USB speakers (no sound card), my speakers are on, volume at a normal level, no music or sounds playing. It may or may not coincide with planes or helicopters flying overhead. All of a sudden, I hear voices. Distinct, at a low volume, but I can make out words. It happens for short periods and fades out after seven or eight seconds. I know it is coming from the speakers because when it happens I lean in close to make sure. It has happened several times, and most recently I heard a man saying “I didn’t think he would go across there, but he did.”
What is the likely source of this? Police? Helicopters? Dead people? Are my speakers crappy, and that’s why this is happening? Are my speakers somehow amplifying a signal they receive, or does the signal directly affect the speaker cone to make a sound? Does this happen to anyone else?
I might also posit your neighbor’s cordless phone, cell phones, and CB radio from nearby truckers. The last one happens to us at work occassionally on our short range two-way radios.
I was totally freaked out once when voices started coming from a baby monitor–and nobody else was in the house. The conversation was of, um, a distinctly ‘personal’ nature, no less. When I looked outside, I saw one of my neighbors sitting on her porch chatting on a cordless phone … I wasn’t sure which of us would be more embarrassed more if I brought it to her attention, so I just switched the monitor to a different channel.
I would expect a baby monitor or two-way radios to pick stuff up, but regular old PC speakers? How are airborne signals affecting what comes out of the speaker?
It’s usually what’s called “common mode interfearance” in which the Rf is picked up by the speaker leads and is detected (demodulated)by the amplifier. This signal, no longer RF, is then sent to the speakers just as if it had been a normal audio signal.
That’s a pretty simplified version.
The problem can often be eliminated or reduced by obtaining ferrite toroids (donuts) and winding the speaker laeds around and through the center of the donut. The more times around the better. It’s called common mode because the interfearance is picked up by both speaker leads and is “common” to both wires.
I’ve been wondering about the same phenomenon, but I’ve always been sure about its source – CB’s in trucks passing by the office. (Which is located at the intersection of an Interstate highway and a four-lane U.S. Highway.)
One question that doesn’t seem to jive with your answer, though. I get the truckers talking loud and clear even when the volume on my amplified speakers is turned completely down. Is this the same phenom, only different? Would I get the same result with non-amplified speakers? Or does the demodulation take place on the back end (speaker lead end) of the amplifier?
It is possible for the speaker leads to act as antennae (ie. for the EM field to induce current in speaker leads that are about half-dipole or dipole length) and for the self-inductance of the speaker coils to crudely demodulate AM signals. Try wrapping the leads in alfoil.