Well apparently there's no rules stating the maximum spl an exhaust can be on a car. An MOT test station can refuse to give you a certificate if they deep it to be excessively noisey however there's no actual figure, this must purely be subjective.
Yeah it cant be 9 decibels, thats pretty much silent. The only way i can see how they would do you is to pull you over and do a spot check on the car, and no doubt they will manage to haul you up on something.
i got pulled in 2003 in my swift with a standard exhaust on - i just had a 30 quid over the top tail pipe from halfords and a police car seen it and pulled me and said my exhaust was too lud while i was stood stationary at the traffic lights - he was in shook when i set off and it hardly made a noise cos it was standard with just a tail pipe bolted over it - i should have complained!! but he informed me it was either 90 db or 95 db cant remember which