Purely anecdotally I used to have a motorcycle that after about a year had the muffler need replacement. I was a poor student and so bought a fake one instead that was just a muffler shaped pipe.

The difference was immediate and extremely noticable, the number of near misses went down a huge amount and the few times people got a little close I could just chuck the clutch in and give it a good rev to scare anyone off.

My biking days are done for now but if I go back I will not be going back quietly.

Why wouldn't you just put a wind-powered vuvuzela on the front instead? Then it's only being obnoxious when you're in motion and it's pointed in the direction that you want people to hear you instead of the opposite.

Cars are isolated? On a regular basis I drive 5 different cars, various makers, I can hear a Harley just fine in either.

And from every parked car where a driver has a conversation on speakers I can hear it so well they might as well have had it outside of their car. New Teslas, BMWs.

Cars are not sound proof.

Depends on the car. Ever rode in a Mercedes S-Class at 250 kph on the Autobahn? Unnoticable, except of the fast whooshing by of overpasses, or such. (As front seat passenger)

No, I have drive in a class and it’s not sound. Not on the Autobahn though ;) I guess I should have clarified.

Almost all affordable cars are not sound proof.*