Idk, I'm curious how common it is outside of Germany specifically. I've heard that Germany has exceptionally strict noise restrictions.

Yeah, immission isn't a word I've ever heard before so excuse my ignorance. Seems to correlate with how things are measured around here, where basically you take a historic ambient sound level and then the adjusted sound level after a source is introduced. For development, obviously doable. For people traversing the local roadway? Literally impossible. In fact, where I am from, the vehicle traffic associated with a use is typically exempt from their land use application because it's just not feasible to get an accurate representation. It's also not feasible to have a business put a "No popcorn tunes" sign up, and then actually enforce it.

Do you have any info on how this 35dB rule is enforced? Genuinely curious. My familiarity with the technology is through the testimony of experts in a quasi-judicial setting, and so I am far from an expert, but what I've heard with enough frequency is that it's difficult if not impossible to pull out vehicle noise from ambient noise.

In that vein, there are restrictions based on the duration of noises and their level. Basically, a car door slamming is instantaneous. The backup alarm on a truck is constant. But yeah, unrelated. I just don't know that you can use decibels to limit the motoring public.

And I say all of this being so staunchly against aftermarket tunes making your exhaust sound like you're running rich. Shit is stupid. I say this having, at a point in my life, drove a car (factory stock) that had a loud exhaust. And so I think if you're concerned, the best place to make a change is in regulations associated with vehicle exhausts. Neuter pickups and cars if they can't cc comply. You need noise to get horsepower and torque? Too bad, muffle and restrict your pipes, and if you can't make your vehicle strong enough, maybe make it smaller.