Show HN: Keytap2 – acoustic keyboard eavesdropping based on n-gram frequencies 6 years ago
You may find this interesting - the technique has been used before to do unsupervised recovery of keystrokes:
"Cracking Passwords using Keyboard Acoustics and Language Modeling"
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.186...