The tap code, sometimes called the knock code, is a way to encode text messages on a letter-by-letter basis in a very simple way. The message is transmitted using a series of tap sounds.
Here is the conversion table:
The secret letter is always encrypted with two blocks like this (... .) -> L. First you count the row "taps" and then column "taps". So, to get letter L you need 3 taps and then 1 tap.