What's up with my speakers?

November 29, 2008 4:26 AM

My stereo is off, unplugged even. I am not using my cell phone, neither is my adjacent roommate.. why do I occasionally hear loud conversation, almost sounding like walkie talkie two way radio communication, coming from my speakers?

This is very rare, and has only happened 4-5 times in my lifetime.. and not where I currently live but once. But for reasons I can't imagine, I on that rare occasion will start hearing what sounds like two way radio convesation. But the stereo is off, I've even unplugged it during these incidents, disconnected the speakers even if I remember correctly (and there's a very good chance I don't).. this has always bugged me, why/what is that?

posted by mediocre to technology (6 answers total)

This previous post might be of help.
posted by spockette at 4:37 AM on November 29, 2008

Yes, as suggested in spokette's link your speaker wires are acting as antennae and picking up transmissions from somewhere else. Depending on your location, your emergency services might be using communication equipment that could be picked up like this. Many forces around the world have switched to digitally encoded transmissions but some still use analogue broadcasts, especially in the USA. Another possible culprit is a nearby cordless phone: again, modern versions transceive digitally, but older ones still use analogue.

To stop it, you need to stop your speaker wires acting as an arial. The easist ways to do this are to either:
a) Change the length of the wires. Signal gain is strongest when the arial's length is a whole multiple of the signal's wavelength; you want to avoid this.
or
b) Attach ferrite beads to the wire (to change its impedence, IIRC).

You can get ferrite beads for next to nothing from an electronics store like Maplin (um... or radio shack in the USA?). Alternatively, you can pull apart the thick collar on the end of an unneeded USB cable - you'll find a ferrite bead or ring in there.
posted by metaBugs at 6:41 AM on November 29, 2008 [1 favorite]

It seems that you need to rectify your problem with rectification. metaBugs' advice is good.

If it's only happened a handful of times, I'd be curious what's causing it. If you had a ham radio operator next door, I'd suspect you'd have trouble more often than a few times in your life. (And here in the US, at least in my parts, only the police are moving to digital systems; most other agencies are remaining analog, either because of the obscene cost ($2-4,000/radio) of digital, or because it works horribly when you're wearing an oxygen mask for firefighters. So it could conceivably be something like a fire truck, or really any agency using 2-way radios.)

Yes, I'm identifying the technical term solely so that I can make a corny pun.
posted by fogster at 7:00 AM on November 29, 2008

I usually chalk this up to people with modified CB radios which put out a lot more power than they're supposed to. At the same time, the amplifiers they use are usually crappy quality, and throw out garbage RF all over the place. The times this has happened to me, the chatter I hear briefly sounds like CB stuff.
posted by autojack at 10:20 AM on November 29, 2008

I was just listening to a DVD about this topic, because it comes up in doing electrophysiologic studies. MetaBugs is right that the speaker wires are picking up EM interference from the air and transferring it to your loudspeakers.

The longer your speaker wires, the more prominent this problem will be. One way of reducing the problem is to use shorter speaker wires. Another way is to use twisted speaker wires - the twists cause some of the EM energy to cancel out. Another way is to use shielded cables.

I am not sure ferrite beads are going to work for passive EM interference like this, but it is worth a shot.
posted by ikkyu2 at 11:48 AM on November 29, 2008

Another fix that sometimes works is to twist your speaker cables into coils -- just wind them loosely around your hand and use a cable tie to hold the coils in place. This produces a sort of "back current" (negative inductance) that will decrease the inductive effect that is producing this sound in the first place. This may not always work, but it is a trick that we used to use to reduce radio interference when I worked for British Telecom.
posted by Susurration at 7:42 PM on November 29, 2008 [1 favorite]