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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)?

Oliver Salzburg
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1 Answers1

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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.

Several years on, I no longer think ferrite chokes will do anything for the noise. The original answer is left below for posterity. A shielded cable may help, but most speakers won't support such a thing. Modern mobile phone technologies don't really cause these noises anymore.

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.

image

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.

Bob
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