These type of bongs have a percolator which acts as an additional and secondary water chamber .
It adds up a quite bit of experience as the smoke is being filtered through many chambers and get colder and smoother at every time. You can also add extra diffusers in these bongs which will maximize the filtering and hence more cold and smooth
There are two most commonly used percolator bongs such as tree and dome percolators
These percolators might use 1 to 8 individual flow paths for the smoke to enter into water and the number of paths depend on the manufacturer of the bong. When the design of a percolator goes smaller and complicated , it is much difficult for a manufacture to make it. The number of holes/slits and the size of the slit are equal to the inside diameter of the upstem to make the air flow equivalent.
These percolators direct the smoke through a single encircling cylindrical piece. The number of slits and their size depends on the inside diameter of the upstem just like tree percolators