Countermeasures
This kind of cryptanalysis can be defeated by generating sounds that are in the same spectrum and same form as keypresses. If you replay sounds of actual keypresses, it may be possible to totally defeat such kinds of attacks. It is advisable to use at least 5 different recorded variations (36 x 5 = 180 variations) for each keypress to get around the issue of FFT fingerprinting. Alternatively, white noise of a sufficient volume (which may be simpler to generate for playback) will also mask the acoustic emanations of individual keypresses.
Read more about this topic: Acoustic Cryptanalysis