Dnl65 From United States of America, joined Jun 2001, 79 posts, RR: 0 Reply 1, posted (12 years 4 months 4 weeks 13 hours ago) and read 1234 times:
Stage 3 identifies the allowable noise levels for U.S. Certificated airplanes. The actual definition which is quite complex is located in FAR Part 36. The Airport Noise and Capacity Act of 1990 required that all U.S. Civil aircraft weighing over 75,000 GTOW meet Stage 3 requirements by 1/1/00. Although all other aircraft have their noise levels identified only those over 75,000 pounds are required to be stage 3.
Aircraft have to meet noise limits on takeoff, on approach, and "sideline". The measuring point for takeoff is 6500 meters from the start of takeoff roll, on the runway centerline, and the stage 3 limit for a four-engine aircraft weighing 850,000 lb or more is 106 EPNdB, while the limit for a twin-engine weighing 300,000 lb would be 95 EPNdB. Seems to me if you don't quite meet one criterion you can make up for it by beating the other criteria with something to spare.
I guess somewhere in FAR 36 you'll find a definition of EPNdB that's about 200 times more detailed than you wanted; it's something about weighting the various frequencies so that an Effective Perceived Noise (?) Decibel is a better measure of how noisy something seems to the human ear.