Hotdog,
I got you now. A lot of people confuse "engine" noise with "aircraft" noise. Different airplanes with the exact same engines will most likely not have the same certified noise values.
When I see magazines that say things like "this engine is "Stage 3" or "Chapter 3" I laugh. No engine is noise certified, it is the entire aircraft configuration that gets noise certification. Different aircraft perform differently as to thrust required at cutback, speed at the sideline point and thrust required on approach.
Engines with a larger bypass ratio will undoubtably be spinning a lot slower to produce the same or greater thrust as a lower bypass engine. This is the killer on approach. Cutback is mostly jet noise with very little fan component in the spectrum.
Things like exhaust mixers, acoustic liners and the newest tweek of the week.....performance robbing "chevrons" help in reducing the jet noise, well the liners in the exhaust only work for about a month and then they fill up with so much crap that they basically do nothing. Repairs to the liners also reduce their ability to reduce the source noise.
The best way to reduce noise on an already operational aircraft is to improve the performance of the aircraft(ie. improve L/D) Anything that can be done to reduce required thrust for any aircraft will have a large effect on the noise numbers. For the most part we in the noise world have hit a huge wall as far as technology that we can throw on aircraft to reduce noise. We used to get huge reductions and now we scrape for even tenths of a dB.
FYI - The A380 design increased fuel burn in cruise by 1% by tweaking the wing design towards lower thrust required on approach so as to make their intended QC catagory at Heathrow. Nice of the Bus to design a plane that is cheaper for the operator to operate but costs the rest of us in cash(increased ticket price) and environmentally.
