You should re-read the article. It didnt reject communications via tapping is NOT POSSIBLE. It is saying it is unrealible and can confuse the messages as some tappings are hard to distinguish. Next time, try to use a successful example to back your argument. The article is stressing the fact that Morse code cannot be use to transmit message reliably over a pipe via tapping. It is not refuting doing this communication is IMPOSSIBLE. Explain to me how you are going to do flags over pipes or walls. Stay within the context of discussion.