I don't know how your windows are made, but you can't really use glass to gauge the sound level of noise. If it is in anyway loose, it acts like a very rigid speaker diaphragm. It moves back and forth with any sound, since glass basically doesn't flex. What you're actually measuring is your own ability to perceive its movement.

As far as fining motorists for horn usage, if it works like everywhere else in the US there is no way they could ever actually make good on the fines. As long even a small percentage of people requested a court hearing (doesn't matter if it is a jury trial or something else) the police officer would basically spend 1 day writing fines, the remaining days in the year just sitting in court.

I'm unsure though, maybe NYC has a provision allowing it to issue fines you can't contest.