Mine are a 4" tube, 2.5" perforated inner which then has pot scourer donuts stretched over it. Sorry dont have a pic of the scourer setup to hand. Dont use steel "wool" as it can burn, whereas the scourers are more like wire mesh. I got the scourers from Woolworths, 3 to a pack were about $0.60 I think, needed less than I thought I would. Anyway with all this my system is pretty quiet but I think the cats do more than the pipes....
Modena's progress update