Electronic devices can be surprisingly leaky, often spraying out information for anyone close by to receive. [Docter Cube] has found another such leak, this time with the speakers in iPhones. While repairing an old AM radio and listening to a podcast on his iPhone, he discovered that the radio was receiving audio the from his iPhone when tuned to 950-970kHz.
[Docter Cube] states that he was able to receive the audio signal up to 20 feet away. A number of people responded to the tweet with video and test results from different phones. It appears that iPhones 7 to 10 are affected, and there is at least one report for a Motorola Android phone. The amplifier circuit of the speaker appears to be the most likely culprit, with some reports saying that the volume setting had a big impact. With the short range the security risk should be minor, although we would be interested to see the results of testing with higher gain antennas. It is also likely that the emission levels still fall within FCC Part 15 limits.
I've confirmed this on an 8 non-pro AND discovered that when you're around 960 KHz you ALSO can detect the haptic clicks! I am suspecting that they are running a class D amp with the PWM at 960 KHz, which would be 20x 48khz-right on the mark for a class D amp. Video demo of both. pic.twitter.com/EbodZS11kB
— iCarly Shinji (@jonny290) September 3, 2020
The phenomenon of electronics leaking unintended information in magnetic waves has been knows since WW2, and it even has a codename, TEMPEST. If you want to learn more about it and do some experimentation, there are a number of open source software toolkits you can play with.