Cell phones and electromagnetic emission


I’ve noticed that whenever a cell phone is next to some device such as CD-Player or PC speakers whenever the phone starts to ring or a SMS message is being received, some kind of sound is produced on the earphones/speakers as well. I know it has to do something with electromagnetic induction, but could you explain me exactly what is going on there and how/why this sound is produced?

Cell phones emit radio waves. The wires and circuit traces in electronic devices act as small antennas, which pick up these radio waves. The typical AM receiver circuit is just a diode and a capacitor, and most electronic devices have lots of diodes and capacitors in them, making them natural receivers of radio noise. Amplifiers are particularly vulnerable, since only a small amount of noise signal coupled into the audio path will get amplified into something that you can easily hear.

To expand on the answer given by e_c_g, the reason your phone causes these problems only when it rings or you receive a text message is because of the way the cellular system works. When someone calls you or sends an SMS, the cell towers in your approximate vicinity all send out a signal that tells your phone that a call is coming in, or that a message is being sent. When your phone receives this signal, it sends back and acknowledgment that 1) tells the cellular system that your phone is on and 2) identifies where your phone is in the network. It is this acknowledgment signal that is causing the effects you hear.