That is part of the article. I want to ask about step 4. I know the basic theory of how stationary wave is formed (superposition of incoming and reflected wave) and also basic concept about stationary wave in open and closed tube, something like this:
But I don't know the reason why in step 4, the speaker must be placed at angle 45o, not pointed directly into it. If I imagine it, putting the speaker directly in front of the tube will also produce reflection and possibly stationary wave.
Why should be the speaker placed at certain angle? And must it be 45o, or can it be some other angle such as 60o?