The length of the chamber controls the frequency of sound that it will best attenuate, rather like organ pipes for low notes are longer than those for high notes. It's the same science, with different numbers.

If you want to control as much of the noise spectrum as you can, then you need additional chambers, and they need to be of different lengths that are not exact multiples of each other.

There is no substitute for cubes, as BB says. It's the expansion ratio from the bore of the barrel - or the hole in the baffle - to the internal diameter of the tube that controls how much reduction you will get. Making the hole in the baffle as tight a fit to the pellet size as your manufacturing ability will allow is a big factor.

This is my most successful one to date. http://i214.photobucket.com/albums/c...MG_0111res.jpg
It has four chambers and is a reflex design; you can see the little ports at the back where much of the air vents.

PS I meant to add, it's close to 70mm diameter.