I've been wondering why there isn't more give and take for the development of more fuel and environmentally friendly engines. The objective for new engines seems to be quieter, using less fuel, creating less pollution, with lower CO2 emissions. It seems to me it would be more productive to allow a little give and take. For example, it's fairly well known to aviation geeks that ultra-high bypass engines and/or unducted fan engines are more efficient, but have a higher noise threshold. While that can be overcome inside the cabin with active noise canceling (I think Saab did something like this), outside noise is still a problem. So here's my idea: why not, for the sake of the environment, and fuel savings, allow a "noise exception" for environmentally friendly engines for the near term allowing manufacturers to put units in production, get the bugs worked out and work on making successive quieter generations. If it's really important to lower emissions as many believe it is, isn't it worth a little noise to save the world? We'll have to sacrifice if we want to save resources. (Insert standard rant here about people who move next to airports and then complain about noise... yeah PDKwatch I'm talking about you.) As we all know, standard turbojets have become MUCH quieter with maturity of technology (just compare a JT3D fuel-to-noise converter with a CFM56-7), so why wouldn't the same apply for UHB/UDF technology? So my bottom line is why has no one proposed this, or why would this never work as a proposal?