The problem of engine noise has always been associated with aircraft. Increases in engine power have given rise to increases in noise and the indications are that the increasing power trend will continue even more rapidly in future. High noise levels are responsible for psychological and physiological damage to humans and can also cause structural damage to aircraft; this has led to limits being set on maximum noise levels of aircraft by airport authorities and it appears that these limitations will be even more severe in future. The unit that is commonly used for measuring the noise annoyance level is the perceived noise decibel (PNdB). A PNdB is a measure of noise annoyance that take into account the pitch as well as the pressure (decibel) of a sound.
The figure compares the noise level bands of various jet engine types (a busy restaurant will be 75-80 PNdB).