From MAKE 30: How Does the “Sound Sucker” Work?

In the current issue of MAKE, Vol 30, we have an easy project by William Gurstelle for building a “sound sucker” using a mug, Jello, and hollow stirring straws. The instructions for making it are shown here (click thumbnail image below).

In the article William says:

Using this “sound sucker” device allows you to experience a curious sensation: it’s as if sound is not only being blocked, but actually sucked away from your ear.

The sound sucker works on a narrow range of frequencies. My testing showed it most effective a few cycles to either side of 660Hz (depending on the amount of gelatin), and the effect is most noticeable in a room with a wide spectrum of ambient noise frequencies.

We don’t know how the sound sucker works. Can you explain this acoustic phenomenon? If you can, post your explanation here.