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?
back “in the day” of 2 and early 3g phones, the signal spectrum needed a significantly more powerful signal to get the phone to ring.
your phone would listen for a radio signal telling it there was an incoming call.
it would then turn on it’s onboard amplifiers, and start to amplify that signal, as well as to send a signal to the nearest cell tower saying “i’m here, and i’m answering”.
the series of pulses you would hear on speakers was the interference from those outgoing signals.
modern day mobile phones (4 and 5g) firstly use a much higher frequency spectrum, so any pulses you would possibly hear would be significantly faster, and higher pitched, and secondly need much less powerful signals, so if you did hear them, they’d be a lot quieter.
plus new mobile phones transmit much more tightly attenuated signals, which modern speakers can be fairly easily shielded against by simply including a ferrus coil somewhere inside.
tl;dr modern phones make much less electrical noise when they answer, and modern speakers are better sheilded.
Before 5G there was (and still is) 4G – before 4G there was 3G – before 3G there was GSM (which is part of the family of 2G). A feature of GSM is that it would send a strong signal to sync up with the towers in the “cell” as the GSM technology relies on Time Division Multiple Access (TDMA). It’s a little difficult to ELI5 this but basically until the call signal is sync’d up your phone needs to broadcast through all available time slots. Once it is sync’d with the towers then it sends its packets via precisely timed microbursts:
>In the GSM system, the synchronization of the mobile phones is achieved by sending timing advance commands from the base station which instructs the mobile phone to transmit earlier and by how much. This compensates for the propagation delay resulting from the light speed velocity of radio waves. The mobile phone is not allowed to transmit for its entire time slot, but there is a guard interval at the end of each time slot. As the transmission moves into the guard period, the mobile network adjusts the timing advance to synchronize the transmission.
>Initial synchronization of a phone requires even more care. Before a mobile transmits there is no way to actually know the offset required. For this reason, an entire time slot has to be dedicated to mobiles attempting to contact the network; this is known as the random-access channel (RACH) in GSM. The mobile attempts to broadcast at the beginning of the time slot, as received from the network. If the mobile is located next to the base station, there will be no time delay and this will succeed. If, however, the mobile phone is at just less than 35 km from the base station, the time delay will mean the mobile’s broadcast arrives at the very end of the time slot. In that case, the mobile will be instructed to broadcast its messages starting nearly a whole time slot earlier than would be expected otherwise. Finally, if the mobile is beyond the 35 km cell range in GSM, then the RACH will arrive in a neighbouring time slot and be ignored. It is this feature, rather than limitations of power, that limits the range of a GSM cell to 35 km when no special extension techniques are used. By changing the synchronization between the uplink and downlink at the base station, however, this limitation can be overcome.
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.)