There are two O-rings that hold the tubing in place so it doesn't pull out or get sucked further down in which case it would no longer conduct air.
Nifty little air vortex. It also keeps the water level high so that there's no noise from the water coming over the baffle.
It's very quiet now and only took me 15 mins to make after putting up with all the noise for nearly two years. I already had the acrylic pieces from an earlier project and only needed to drill holes in the tube.