[1][2] Because most modern computers, especially laptops, have built-in microphones and speakers, air-gap malware can be designed to communicate secure information acoustically, at frequencies near or beyond the limit of human hearing.
In 2014, researchers introduced ″AirHopper″, a bifurcated attack pattern showing the feasibility of data exfiltration from an isolated computer to a nearby mobile phone, using FM frequency signals.
[7][8] Later in 2015, researchers introduced "GSMem", a method for exfiltrating data from air-gapped computers over cellular frequencies.
The transmission - generated by a standard internal bus - renders the computer into a small cellular transmitter antenna.
OOB-CCs are not as high-bandwidth as conventional radio-frequency channels; however, they are capable of leaking sensitive information that require low data rates to communicate (e.g., text, recorded audio, cryptographic key material).