I’m sure many people have experienced the phenomenon wherein a cell phone that is pointed at PC speakers causes some sort of electromagnetic interference (manifested in the form of the speakers “clicking” rapidly) right before it rings or an SMS is received.
This routinely happens to me when my phone lies in front of my friend’s computer, but never with my own PC. Also, my friend’s phone never causes the “clicking” to happen, whether on his computer or mine.
Why is this? Shouldn’t it be a universal process? Admittedly, we use a different model of handset (Nokia 5210 for him and 2100 for me) and our service providers are different (though what that has to do with it, I’m not sure). Does this matter?
Also, is it right that this intereference is precisely the reason we’re supposed to switch off electronic devices (especially phones) on board airplanes?
There are essentially two different types of speakers, amplified and non-amplified. The non-amplified ones are powered completely from the sound card. The amplified ones have their own built in amplifiers. A cell phone is more likely to interfere with amplified speakers than non-amplified ones, simply because the signal levels in the amplified speakers are so much smaller. You have to generate quite a bit of noise to have it be audible on just a passive speaker.
Even in similar systems, the exact layout and length of wires makes a big difference. All of the wires and circuit board traces act as little antennas, and their length and configuration makes them effectively be a bunch of little tuned antennas. When the frequencies of the cell phone happen to match up with the frequencies of these little tuned circuits then more of the signal gets coupled into the speaker system.
One of the reasons you have to switch off a cell phone on a plane is that it can interfere with systems on the plane (it’s caused at least one loss of communication that I’m aware of and screwed up some instruments, so I’m not sure why there’s really any debate about it). Another reason is that it wreaks havoc on the cell phone company since the phone will contact several cells all at once as you fly overhead, which apparently causes their computers some grief due to all of the extra processing involved.
No, this isn’t right. All phones use the frequency bands allocated to the service provider under the GSM standard used in your region. In Europe GSM 900 and 1800 (the phones automatically switch between the two), in the US and Asia I think 1900Mhz.
The strength of the signal and the level of radio emmissions generated by different models of phone could vary though.
By the way, a similar thing sometimes happens on a smaller scale when switching a light off in the same room as a pair of amplified speakers. If the lightswitch arcs (generates a small spark) it can cause the speakers on the other side of the room to ‘pop.’