Why does my cell phone screech when it is near my computer?

David Grier, chair of the physics department at New York University, dials up some possible answers to this mystery:

It sounds like a case of electromagnetic interference, or EMI: radio waves emitted by one device causing undesirable behavior in another. Virtually every piece of electrically powered equipment acts as a radio transmitter, whether it is supposed to or not; the changing electric currents running through these devices naturally radiate electromagnetic waves. This radiation is an inevitable by-product of harnessing electricity to do useful things, analogous to the clanking and clattering of traditional mechanical devices. Computers are particularly “noisy” because they rely on rapidly changing currents to act as clock signals that coordinate their calculations.

One possible explanation is that your computer unintentionally emits radio waves in the range of frequencies reserved for cell phone communications, typically around 800 megahertz (millions of cycles per second). If the signal coming from your computer were strong enough, your phone could mistake it for a cell phone transmission—albeit an indecipherable one.

Another possibility involves a deeper connection between your two devices. Just as changing currents generate radio waves, radio waves induce electric currents in conducting materials—which is how a metallic antenna allows a radio to detect signals transmitted by radio stations. The radio waves emitted by your computer may induce currents in the amplifier that drives your cell phone’s speaker, which would cause it to produce random squeaks and squawks. (In 1975 computer pioneer Steve Dompier cleverly commandeered this effect, with more tuneful results: he programmed his PC, a MITS Altair 8800, so that its EMI would play the Beatles’ “The Fool on the Hill” through a nearby AM radio.) There is no way to stop electrical devices from generating radio waves, but keeping spurious waves under wraps will curb EMI. Most electronic devices are housed in cases—either made of metal or coated with a conductor—that trap these electromagnetic waves, but holes in the cases and thin spots in the coating allow some waves to escape. Usually the leakage is so small that it just affects objects very near the source, which is why your cell phone only acts up right next to your computer.

Source: The Scientific American, May 2008, page 108