Funnily enough, I think this may be *less* enforceable than the previous rule. You can measure sound in decibels, but it has to be relative to something; usually, this is defined as a barely audible 1000hz sine wave (aka dB SPL). This works fine when you are measuring something like a rock concert or a power tool -- you know what SPL pressures will cause hearing damage, so measuring at the location of the audience / user / bystander allows you to easily determine whether hearing protection needs to be worn or the tool muffled. But, this legislation does not specify a distance that the measurement must be performed at -- sticking a db meter into a tailpipe on any vehicle will give readings above 60 dB, likely 95 dB as well. Additionally, it does not specify a weighting, which determines what frequencies are prioritized; Human ears do not hear all frequencies equally, if you want to determine how loud something is, your measuring device should have the same frequency response as the human ear. Ears also emphasize difference frequencies depending on how loud the sound is, see A vs C weighting. Additionally, since you're performing this over some time, how long is the measurement window? A good meter will offer 'fast' and 'slow' options for time average. I like building and playing with big speakers, and part of doing that responsibly is making sure you don't piss people off or lose your hearing; I've used sound meters at gigs to make sure we kept things to a level where we were not annoying neighbors. In one case, it was neighbors a mile+ away on the other side of the valley, we had SPL levels to stay under that depended on temperature and humidity.
So, a cop measures your car, and it fails the test. Is that A weighted? C weighted? Fast or slow? At what distance? When was the meter calibrated? These are all questions that could be brought up in court to fight any tickets coming out of this law. I am not a lawyer, but as I recall this law should be unenforceable -- There's not enough information available so that a citizen can ensure compliance with it. Obviously, neither did the previous language, but it did explicitly say that it was up to the cops judgement, and if you don't agree with it then you can argue it in front of a judge.
So it's a dumb bill (it's not a law yet, thankfully), but I will say that there is a problem that needs addressing here: There has definitely been an increase in large groups of bikes zooming around urban-ish areas in the past year or so. I actually have only seen / heard of bikes and ATVs doing it, not cars. It's not the usual Harley folks, it's usually 10-50 guys on dirt bikes and ATVs doing wheelies and speeding. They run red lights, make an obnoxious amount of noise, and generally are a nuisance. The person who authored the bill represents a district down in Brooklyn, which further makes me thing that's what it's supposed to address. But I also don't really see how changing laws about exhaust loudness is supposed to make a difference: They're already breaking several other laws, and if the cops were interested in doing their jobs and going after them they would have by now, but it's probably better they don't since I don't think any of the riders in these groups are the sort to pull over all polite-like, nor are they necessarily registered / plated. I'd rather just live with some people getting their kicks in a loud and obnoxious way than have bystanders put at significantly more risk by police officers in cars chasing bikes and ATVs through the city. Also, the massive difference between the car / truck requirements and the bike requirements are ridiculous. Just today I was out with my lovely partner getting some coffee and pastries and had a group of four Harleys with straight pipes roar past us in town. Choking fumes, chest pounding badly engineered rumble, speeding down an urban street with crosswalks and pedestrians, the whole bit. That experience was vastly more irritating than the guy in the new STI with a loud aftermarket exhaust: sure, we noticed it every time it moved stoplight to stoplight, but it wasn't oppressively loud, it didn't stink, and he kept his foot out of it until he got out of town proper.