Science Questions

Why do speakers make an odd noise when near a ringing phone?

Sun, 3rd Apr 2011

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Question

Anthony Lewis, Facebook asked:

Why do all speakers make the same odd noise when they are near a ringing phone & intercept the signal?

Answer

Dave - The phone is sending out a signal to all the nearby base stations, essentially telling them where it is, and I think possibly also to allow the base station to characterise what direction it should send the the signal in, and all sorts of other things like that.  It does it just as it picks up a signal and also does it randomly during the day so that the base station knows where it is.

Chris - So you get an induced current in the speaker wire and off it goes!

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The phone is basically a radio transmitter, operating at either around 800,900 or 1800,1900 MHz. The transmitter is turned on in pulses to send the compressed digital voice/data to the base station at a rate of 217 Hz. The transmitted signal is picked up by the wiring inside the speakers, and the input circuits have to be non linear, to recover the 217 Hz pulse train from the high frequency signal. If the input has a good filter, and the wiring inside is properly shielded then you will not hear anything. If there is no non linear circuit to demodulate the signal you will hear nothing, even if the signal is there. SeanB, Tue, 5th Apr 2011

I think only active speakers are that sensitive to the emissions from a telephone. A rectifier in the active speaker's circuit probably acts as a radio detector. The output of that detector is then amplified, along with the weak audio input from the radio, TV, etc. Phractality, Wed, 6th Apr 2011



To me, that seem a bit more likely. The impedance of (most) speakers is rather low, so I'd be a bit surprised if the RF signal could couple enough energy into one to produce an audible effect.

I wonder if the OP still detects the same problem when all audio amplifying devices, including any inside speaker enclosures, are turned off?

Geezer, Wed, 6th Apr 2011



That makes total sense - it explains why you get the problem at your PC but not at home with the music system imatfaal, Wed, 6th Apr 2011

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