acoustics - Why speaker beeps when a cellphone is kept near it?

Once I had kept my cellphone near a computer speaker. After some time, a call came over my phone. Strangely, the speaker started humming and beeping till I received it. Why did the speaker started humming when the call came?

Answer

According to this article (which links another article that has since become a stale link), the issue is that during ringing (and other communications between the phone and the tower) there are pulses of (relatively) high power RF transmission by the phone.

Normally, this should not matter. But when an RF signal interacts with a non-linear circuit element (for example, anything with a diode in it) it is possible that you detect the "power envelope" of that signal. This is of course how the "detector" element of many radio receive circuits works.

Many computer speakers contain an amplifier - making it well suited both to detect and amplify the RF power waveform, and making it audible. According to the article linked, the conditions are:

The type of interference can occur if the following things happen together:

1) a pulsing radio transmitter,
2) with relatively strong power,
3) in very close proximity,
4) to a non-linear circuit element.

The non-linear circuit element is usually some sort of solid state device such as a transistor or diode. If the non-linear element is subjected to a strong pulsing radio signal, it will act as a rectifier and "detect" the pulsating waveform, i.e., convert the pulsations from a radio frequency to an audio frequency (if the pulsation rate is in the pass-band of audio frequencies.) For example, a hearing aid consists of a microphone, an audio amplifier and a small speaker. If a strong pulsating radio signal impinges upon the first transistor amplifier stage, the transistor will be driven into its non-linear range and detect the pulsations. If the pulsation rate is in the audio frequency range, the rest of the hearing aid amplifier will amplify this and deliver it to the speaker, to the great annoyance of the hearing aid wearer.

I have also heard it said that the signal you are picking up is actually the power being drawn by the device - there are significant (low frequency) fluctuations in power that result in near field magnetic induction. If that is the case, then RF shielding will not help. I may try to do some experiments and will report back...