and the difference is personal opinion. We had anti-noise bylaw removed because of 2 factors, the limit was too high for most people (96DB) and because it's really difficult and expensive to monitor. They need special noise detectors and they don't have lots of them so they only used them in areas where most people complain and it resulted in zero change while increasing costs.
My motorcycle is loud. Louder than any stock car but it's far from 96DB yet it's also quieter than a lawn mower, or a weed whacker, or a plane so what do you about those noises?
It's an easy fix. Vehicles are whatever volume they are from the factory. Then just ban modifying them in any way to become louder.
An absolute decibel limit doesn't need to exist. You're just not allowed to deliberately increase the volume of your vehicle above what it's natural level is.
Loudness is not subjective. You can use a cheap device called the phone in your pocket to measure decibel readings come up with a baseline for every car and if it's louder than that baseline then it's not allowed. Also it'll be pretty obvious if it's too loud because the person driving the car will be a chav