Well known since at least 2004 https://web.eecs.umich.edu/~genkin/teaching/fall2018/EECS598-12_files/kbdacoustic.pdf https://web.eecs.umich.edu/~genkin/teaching

Well known since at least 2004 https://web.eecs.umich.edu/~genkin/teaching/fall2018/EECS598-12_files/kbdacoustic.pdf https://web.eecs.umich.edu/~genkin/teaching/fall2018/EECS598... https://www.nytimes.com/2004/12/12/magazine/acoustic-keyboard-eavesdropping.html https://www.nytimes.com/2004/12/12/magazine/acoustic-keyboar... More recent one that works well over VoIP: https://dl.acm.org/doi/10.1145/3365366 https://dl.acm.org/doi/10.1145/3365366