ABSTRACT
There has been a considerable decrease in noise levels from individual aircraft over the past 35 years. The noise levels produced by modern aircraft are about 22 dB lower than those of first generation jet aircraft. This reduction has been achieved as a result of the development of turbofan engines with high bypass ratios, liner technology and turbomachinery source noise reduction. Further aircraft noise reduction will be achieved from improved design of engine systems, decrease in airframe sources and the introduction of noise reduction technologies. Figure 2.1 shows the noise sources in a propulsion system.
