You only cut the weld between the vertical fins. Its about 4mm thick. You don't remove or cut out the entire fin. When you do you release a lot of pressure that was applied to bring the fins together for welding in the first place. I got the trick over the phone from altec engineers before the company went out of business. I believe altec sold the last runs of 811 and 511 horns with the fin welds removed so they really believed in it. tdc