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The FAA has measured external aircraft noise levels and published the results in the Advisory Circular:
AC 36-1H - Noise Levels for U.S. Certificated and Foreign Aircraft
In the appendices, noise levels are listed for Takeoff (TO), Sideline (SL) and Approach (AP).
I understand the concept of Takeoff and Approach, but what is "Sideline"? It is not defined in the document, nor does it show up in a search of the Code of Federal Regulations.
The definitions are specific. (They are defined in FAR Part 36). I'm writing also the Take-off and approach noise definitions just for your information.
Sideline noise: is defined and measured 1476 ft (450 m) from the runway centerline at a point where the noise level after liftoff is greatest.
Take-off noise: is defined as the noise measured at a distance of 21,325 ft (6500 m) from the start of the take-off roll, directly under the airplane.
Approach noise: is also defined and measured under the airplane when it is at a distance of 6562 ft (2000 m) from the runway threshold.
For each of these conditions the maximum noise level is a function of maximum takeoff gross weight, and for the take-off case the limits depend also on the number of engines.
