How exactly do speakers make a noise when a mobile phone, that’s about to receive a text message, is right next to it?


How exactly do speakers make a noise when a mobile phone, that’s about to receive a text message, is right next to it?

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The speakers internal/external cables are picking up the text message signals.

Those cables are essentially acting as an antenna. The speaker is simply doing its job of processing the electrical signals as noise

The radio frequencies on which cellphones operate are near a harmonic of the speaker’s coil. The RF energy excites the coil, inducing a small current which creates just enough of an electromagnetic field to cause the speaker cone to move a little.

When a phone is emitting radio energy, part of it is absorbed by surrounding objects inducing a voltage into them. Multiple conventional cellphones divide the channel among themselves by taking turns transmitting. The speaker effectively gets an irregular square wave signal added to its output, alternating between low when the phone is idle, receiving or waiting, and high when sending data. The speaker is not fast enough to actually respond to the modulated data. The interference would be much less noticeable if the transmission was continuous.


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.