Misc question for the speaker gurus

[Åndhrimnir]

I have an old 2.1 Altec Lansing set which I rarely use. For a while, when I was using them more, I would notice from time to time that they would make a funny clicking/beeping sort of noise. The weird thing was that it didn't sound like random interference - it was very regular, like the sound an old modem makes but more monotone.

Anyways, I was at a friend's house a week ago and we were sitting in front of her computer when the same thing happens to her (logitech something) speakers. Less than two seconds later, she gets a text message. She says all the speaker sets in the house do it when someone nearby gets a text.

I put this to the test. Sent about a dozen texts, from various areas in the house, with her phone on the desk, across the room, out in the kitchen, etc.

Sure enough, it's true. This noise corresponds with the receiving (but not sending) of a text message (and possibly with other wireless transmissions, idk).

Anyways, my question is this: WTF? Why does this occur? I can only assume that it's some sort of interference from the cell tower to something acting as an antenna in the speaker, but does anyone have a more detailed explanation?

 

[ashmedai]

Radio frequency interference. The phone starts talking to the network a bit before the actual call occurs. When the frequency is right, it inductively energizes other circuits. Audio circuits involve amplification, so relatively small currents produced by induction can create rather substantial noises.

 

[Åndhrimnir]

Cool. I suppose that would imply that the reason some communications (receiving text, for example) create interference while others do not is that only certain types of communications use this specific frequency. That's the bit that was most puzzling to me - why it only occurred for certain transmissions. Thanks for the explanation.

 

[obobski]

Radio frequency interference. The phone starts talking to the network a bit before the actual call occurs. When the frequency is right, it inductively energizes other circuits. Audio circuits involve amplification, so relatively small currents produced by induction can create rather substantial noises.

Cool. I suppose that would imply that the reason some communications (receiving text, for example) create interference while others do not is that only certain types of communications use this specific frequency. That's the bit that was most puzzling to me - why it only occurred for certain transmissions. Thanks for the explanation.

yes.

although holy batman how poorly shielded/isolated are computer speakers getting these days
I can sit and text myself silly with my setup at 100% volume and not hear a damn thing

 

[StealthXtreme4]

Radio frequency interference. The phone starts talking to the network a bit before the actual call occurs. When the frequency is right, it inductively energizes other circuits. Audio circuits involve amplification, so relatively small currents produced by induction can create rather substantial noises.

RF Interference is a hell of a thing.

 

[Daishiknyte]

On a lot of car stereos you can notice a similar effect. Also, phones do a fair amount of background communications that can also cause the clicking when you are not actively using the phone.

Shielding is the critical part of this. It doesn't take much to block out the interference but most designs see it as an easy place to cut a corner.