As the regs are written, you can fail one test by a few db, but as long as you cleared other tests with margin, you can be a CAT 3 or CAT 4. I knew Europe takes a tougher stance, I didn't know the details. Since the ICAO regulations were supposed to be the "end all be all" of noise regulations, it has created friction that individual countries take different interpretations...
There is always a lag between new regulations and enforcement. Right now the airlines are trying to set up the politics so that the US never requires retrofit to CAT 4 (from 3). Personally, I'd like to see a phase out in 15 years (or retrofit), but my opinion and $4 gets you a Latte at Starbucks.
Seriously, non-aviation enthusiasts are really annoyed by aircraft noise. How I like to think of the noise regulations:
CAT 1: non-noise regulated. It hurts when these fly over.
CAT 2: If you're in a house, stop your conversation while the plane flies over.
CAT 3: You could yell over the plane if you're talking in the house, but you're not going miss noticing it.
CAT 4: You could yell over the plane if you're outside the house as it flies over.
CAT 5: (proposed, still under ICAO committee) You could still converse outside of the house. This is the first level where the neighbors noisy mower is more annoying than an aircraft.
Please don't get me wrong, I love aircraft. Maybe its because noise regulations would put more $$$ into engine R&D...