My roomie and I recently moved ourselves back into the dorms at our university to begin another fun-filled year of college excitements. This year, our room is one of a very few that has a direct line of sight with the radio station transmitter (I’m looking at it as I type this), which is only a couple of hundred yards away. For those of you looking for the relevant technical data, it’s a 1,000 watt transmitter, with an effective radiating power of 450 watts (for those of you not looking for relevant technical data, this is pretty much gangbusters for a college radio station). Uh, yeah, I work at the radio station. How did you guess?
Anyway, a couple of nights ago, we noticed very faint music coming from my roommate’s computer speakers. After a brief investigation, we determined that we were hearing the radio signals being broadcast from the transmitter. My speakers, on the other hand, are silent.
How is it that this is happening? Speakers obviously aren’t intended to pick up radio signals on their own, yet these are doing so. And yet, clearly not all speakers do it. What up with this? Oh Teeming Millions, help me out!
It’s the speakers’ amplifier that’s picking up the signal. When you’re that close to a transmitter, it doesn’t take much for the signal to find a spot to leak in through and be heard.
Evidently, your speakers’ amp is just that much better shielded against radio signals.
You didn’t mention whether the transmitter is AM or FM, but it’s possible for the amp to be detecting the signal. This is especially easy with AM, where any diode or rectifier in the amp circuitry can act as an untuned AM receiver. As someone else suggested, experimenting with some grounding arrangements might help.
A quick and dirty test is to remove the speakers and see if you can hear it with headphones. The speaker wires may be acting as an antenna. It could also be coming in through the AC power lines. There are devices to filter out radio on AC power lines or speaker wires if that’s where it’s coming in.
I don’t know why this happens, but it’s not that uncommon.
I have a regular weekly show at a bar that’s just across the intersection and about 400 yards away from the main broadcast tower for a group of local radio stations.
The club has a really nice house system for background music, but evidently something isn’t well shielded at all. Every week, we have the bartender kill the house music before we fire up our PA. Every week, as soon as the house music is cut off, music or talk radio starts coming through the house speakers.
It’s not terribly loud, but it’s there and you can certainly make out the music or what they’re saying.
The funny thing is that our PA never picks up any interference, which is most likely due to much the better shielding and durability built into professional PA gear.
One of the first things they teach you in electrical engineering school is how easy it is to make a radio receiver. All you need is a tuned circuit to select a frequency band, something that conducts better in one direction than the other, and something to filter off all of the noise. Most electronic devices have all of these. The trick for engineers is how to make something that is NOT a radio receiver. Amplifiers are particularly tricky since any tiny radio signal that gets coupled into the audio path gets amplified and becomes a large radio signal.
There are two types of computer speakers, those that have amplifiers and those that don’t. The ones that don’t rely on the sound card to have an amplifier to drive them. The bigger the speakers, the less likely they are to rely on the sound card for power, since sound cards usually don’t have very powerful amplifiers in them. If the amplifier is in the sound card, then it’s fairly well shielded by the case of the computer. If the amplifier is in the speakers, then it’s pretty much not shielded from radio waves at all. Combine this with the fact that the amplifier circuitry contains all of the elements necessary to be a radio receiver, then add in the fact that it’s an amplifier (which means that that it will amplify any tiny radio signal that gets coupled into the audio path). This is why computer speakers, especially the ones with their own built in amplifier, are notorious for picking up radio stations, cell phone chirps, walkie talkies, the radio emissions from neon signs, and lots of other radio signals.
The speakers that are not picking up the radio station either don’t have their own amplifier or they have more filtering in their amplifier circuitry to help eliminate radio noise.
If the speakers do not have their own power supply (like a wall wart or a dedicated power line from the computer) then they don’t have their own built in amplifier.
Thanks everyone! The transmitter is FM. The speakers are the kind with separate units are the kind with separate units for treble and bass - two speakers by the computer on the desk, with a subwoofer on the floor. They are plugged into the wall. The actual desktop speakers are fairly small in size, so they may have little or no shielding, which I suppose would explain why they’re picking up the signal so easily. My speakers are beefier, since they handle both treble and bass (I have no subwoofer). Luckily, the signal doesn’t come in very loudly. Does the power of the transmitter have a large effect on all this?
How much radio signal you receive depends on how much power the transmitter is putting out, how far you are from the transmitter, and how good of a radio receiver you are.
You might try moving the speakers around or just turning them slightly. This might make the problem better, but then it could make it worse too.
Ferrites on the speaker cables and power cables might help.
Since it’s FM (and thus higher frequency than the AM band), you might be able to use ferrites to filter it out, as engineer_comp_geek suggested.
Here’s one type of ferrite choke: Radio Shack 273-105. Many computer and wall adapter cables have these to reduce radiated emissions - if you have an extra cable you could steal one from that to try.
Wrap as many turns of the wire as you can around the core of this, as close to the speaker as you can get it. You could also try a capacitor across the speaker terminals, if you’re willing to cut the wire and perhaps do some soldering- a 0.01 uF ceramic capacitor might shunt the signal around the speaker.
You may need to do this to the power input to the speaker in addition or instead of the actual signal wires. If the amp itself is picking up the signal, then none of this will help, though. You can also expirement with moving the speakers and their wires around. Coiling up the excess wire into a tight coil might help.