Ploz,
Actually, the technology and understanding of airframe noise is a new arena for noise reduction. Airframe noise will go down as well as the engine source noise. I will guarantee you that engines will never get quiet enough to where your airframe noise drives your overall noise levels.
If you are anywhere near the noise game you should know that every aircraft in current production meets the 10 dB reduction already so the manufacturers will not have to do anything to meet Chapter 4. 737s that are 20 years old meet the Chapter 4 noise level requirements. Heck, your going to see 727s that meet Chapter 4.
Listen to me when I tell you that you will not see noticeable noise reduction in airport communities for 20 years. They may demonstrate some small reductions, but those reductions are not noticable to any human.
