Explain this exhaust tip phenomenom

Just an FYI, tips can be useful to alter the exhaust sound. My RC had a very "rappy" exhaust. It is running a 10.2:1 comp ratio, headers and true 2 1/2" duals with no crossover and Summit "Turbo" mufflers. On the suggestion of another member, I cut the pipes back 18" and added a 18" long 3 1/2" diameter tips that deepened the exhaust note once it comes on the cam and not nearly as rappy sounding.

Just food for thought is all. :)