Back in the day when mobile phones were around audio speakers, the speakers would emit some sort of crackling/static a few seconds before any call came in. This does not seem to happen these days. What has changed in the phones or speakers or both for this to not occur anymore?
In: Technology
When your cell phone connects to a new cell tower, or when it has a weak signal and wants to re-connect, it sends a few pulses of radio waves, a few hundred times per second, in a very specific and regular pattern. It’s basically the radio version of the beeps and boops that dial-up modems used to make.
If there are any nearby electrical wires, they’ll act like radio antennas and develop an electric charge that’s oscillating in sync with that radio signal. It’s far too weak to matter for most things, but if those wires happen to be carrying an audio signal to an amplifier to play through a speaker, then those pulses will get amplified and played through the speaker too, and it creates that sound you remember.
It’s not hard to block that radio signal – you just have to wrap some metal foil around the wires, and it’ll block the radio waves. These days, now that cell phones are very common, all manufacturers of audio equipment add shielding like that, so you don’t hear that sound anymore.
(And the reason you only hear it when the phone is re-connecting, not every time it transmits, is because most of the time, your phone is sending radio pulses that go on and off hundreds of thousands of times per second – way beyond what your ears can hear, and probably way beyond what the speakers could produce even if you could hear it. You only hear the slow, regular “connecting” signal that’s used to sync up with the cell tower.)
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