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My PC's desktop speakers are periodically producing interference noises induced by my cellphone.

Even if I move my phone away from the actual speakers, the noises are still present with the same intensity. I have to completely move the phone away from my desk to get rid of the noises.

I know many different speakers and phones are affected by this as this has been an issue for me ever since I owned a mobile phone.

Why is the phone causing these noises even though it's far away from the actual speaker? What can I do to get rid of the noises (while keeping my phone on the desk)?

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1 Answer

up vote 10 down vote accepted

The cause of these noises is electromagnetic interference. Common phone radio frequencies can generate a signal in unshielded audio wiring through electromagnetic induction. This undesired signal is amplified by the speakers, producing an audible (and nasty!) noise.

An easy way to stop this noise is to use a ferrite choke on the wiring, to block higher frequencies.*

These should be placed close to the speaker for maximum effectiveness. There are two main types that are easy to apply:

  • Snap on ferrite beads

    These typically come in a plastic casing that splits in half, and can be snapped around your wiring. They are easy to apply.

  • Toroidal ferrite cores

    These come in a toroidal ('doughnut') form. Just loop the wiring through a few times.

    enter image description here

There is no set number of turns, just add more until the interference stops.

Ferrite chokes/beads/cores are easily and cheaply available from many sources. If you have an old USB cable or similar, there are often bulges near the ends. You can rip a ferrite bead off of that. PSUs and computer motherboards also often contain ferrite cores, which can again be ripped off for this purpose. Alternatively, many hobby/electronics/online stores carry them.


*As a side note, higher quality speakers will often include these filters. It's normally the cheaper speakers, especially those sold with/for desktop computers, that get effected.

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This would work for microphones too, wouldn't it? –  BlueRaja Jun 4 '12 at 14:57
    
@BlueRaja I've never personally tried, but it should work in theory. That source also includes more precise methods than "grab a ferrite core and wrap a few turns around it". This is only for the large things like phone interference; there's many smaller sources of EMI that aren't really audible without extreme amplification, or close analysis of recorded audio. –  Bob Jun 4 '12 at 15:13
    
Most microphone cables i've seen are significantly better than your average cheapie speaker or headphone cable, and have shielding. Good headphones use the same sort. –  Journeyman Geek Jun 5 '12 at 4:03
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