I have been working on a quiet core for my Graydon Proline Pipe. The first two go around did not work. Set up #1 was very quiet but acted like a potato in the tail pipe. I did not even test it for sound. I them went to work on set up #2 and I made some modification to open it up. #2 started easy and revved easy because there was on packing in this version I did burn off the Graydon label off the canister. I did the sound test with the stock core verse my core each with 12-suppertrapp disk. The test RPM is 3250, which is the standard for the DS650 at the Oregon dunes. The stock core 97 dB my core 100 dB. Test #3 with a smaller version of my core placed inside of a stock core tested out too 95dB at 3330 RPM. All I need too due is some fine-tuning and it should pass the sound test. All test were done with the sound meter at 20 inches from the end of the exhaust on a tripod setup just like way they due in Oregon.
http://www.oohva.org/pages/sound.html