Well, you can use tap code: https://en.wikipedia.org/wiki/Tap_code https://en.wikipedia.org/wiki/Tap_code Or you can use "Morse tap code" where two close knock

Well, you can use tap code: https://en.wikipedia.org/wiki/Tap_code https://en.wikipedia.org/wiki/Tap_code Or you can use "Morse tap code" where two close knocks represent a dot, and if the gap between them is slightly longer it's a dash. That takes two knocks for one dot/dash but it works and is (was?) used in practice.