ok I'll give you concrete.
I had a full 2.25" system on my '85 Golf. Not cat, no resonator, and a straight through glasspack on the back. It was as loud as hell, but deep toned, I liked it.
Then a cop gave me a ticket for it being too loud/broken. I said look, right there, you can see the muffler, it's all new, there are no holes. It's not broken. I had the receipts in the glove box. He said the law here it was officers discretion on noise levels. No DB meters needed!
So I went back to where I had it done, and they put in a 18" cherry bomb under the car as a resonator. It took a noticeable amount of DB out of the system, and changed the tone a bit. Also it makes a faint "ping, ping, ping" noise under the car at idle, I believe your hearing every cylinder fire. My Current Jetta with a resonator does this too. But I've even heard muscle cars that do this.
So anyways, I took my car down to the Police Station and my receipt, and I said I need somebody to check my car. And the officer working the desk looked at the bill, saw the date on the work done was after the ticket date, it had my car's license plate on the receipt, and he never even bothered to go outside to hear it. The just deleted the ticket out of the system and said "have a nice day"
But yeah it took a noticeble amount of DB out of the system. But I wouldn't call it quiet. It's still fairly loud.
The thing that pisses me off most is my Sportbikes are way louder, and I've never been harassed about the exhaust on them. But I have been given tickets for illegal signal lights, and plate in unreadable location... Meh what are your gonna do, every cop has their own personal pet peeves.