INTRODUCTION The introduction of commercial jet aircraft operations as well as the general increase in air traffic has resulted in an international concern over aircraft noise. The perceived noise level is a single number rating of the noise based upon objective acoustic measurements which is related to the relative subjective response to the noise. The perceived noise level@ as defined in the document@ is based only on the noise spectra measured in octave or one-third octave bands of frequency. As such@ it is most accurate in rating broadband sounds of similar time duration which do not contain strong discrete frequency components. When additional factors such as the duration and the presence of discrete frequency components are to be taken into account@ the effective perceived noise level (EPNL) may be a preferred measure.