Acoustic Frequency Hole - How does this work?

[SuperCass]

Acoustic "Frequency Hole" -- How does this work?

Here is a project proposal that I read:
http://amasci.com/freenrg/audhole.html

I decided to give it a try and I made this project and it works.
But I don't seem to understand why and how this works. Can someone explain this phenomenon?

Thanks!

 

[SuperCass]

Bump!