In the U. S., the EPA specifies dB limits on exhaust noise. 84dB, I believe, but I'd have to look it up to be sure (IIRC, 84dB is what's stamped on my stock exhaust). There are local limits as well. The limits just aren't enforced as a primary offense unless the cop really just uses it as an excuse to check for DUI.

It would be a nice start if national parks started turning them away at the gate, as Glacier NP has talked about doing. I'll laugh my ass off when I see Harley and Harley-wannabe riders doing u-turns at national park entrances.

It's ~90dB and bikers routinely retrofit their bikes with loud, illegal exhausts that are louder than that to show off and because they believe it makes them safer on the road - "loud pipes save lives". 100dB+ is common, that's 10x louder than the already generous limit.