They make you rev your car like around 3-4k and see how loud it is. I was reading about a kid who got pulled over in Cali since he had a Greddy SP and they put noise violation too loud. So he asked the magazine what he should and told him to go to the company and get a like a letter complying it was under 96db and then he told the judge that their was no kind of sound test done. He got away scotts free.