My friend has a db tested thing. Same as the exhaust place uses. He tested my exhaust at 108 db from memory. Then i made up this thing to go just after the cat. Just got a bit of tin cost me $1.75 from bunnings. Cut it to the size of the gaskit. drilled some holes in it and put in in the exhaust. Got may mate to test it again and it was 92db. The exhaust shop tested it at 86db.
Removed it after the test. I have a 3" system.