> The 88s keys already cover almost the entire range of human hearing. Adding just one more octave (a few keys below and above the range) would basically cover it all.

This was surprising for me, I thought no way is this almost the range of hearing.

But a quick calculation showed it to be right: The range of human hearing is approximately from 20Hz to 20kHz which is ~10 octaves (20*2^10 ~= 20k). 10 octaves would need 120keys, so ~2.5 Octaves more than the 88 keys range.