Why did speakers used to make a strange noise when a phone is about to receive a message? Also, why that doesn’t happen as often now (little to no instances)?
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GSM, the old mobile standard, sliced up their channels into time slots such that there were about 1700 of them per second. This allows more than one phone to share a frequency channel. So a phone sending data would be switching its radio on and off at that frequency, which is in the range of human hearing (and thus speakers and amplifiers are designed to work with it). The actual “carrier” radio frequency was much too high to be heard.
Modern mobile phone standards like 3G, 4G, and LTE use different techniques to slice up channels so multiple phones can use them. These techniques don’t include a signal component down in the human hearing band.
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