ESDU 82027 gives a graphical method@ and a computer program (ESDUpac A8727)@ for estimating the free-field values of one-third octave band lateral attenuation. Lateral attenuation is defined as the difference between the under-the-flightpath and sideline free-field levels where the propagation distance of the two sound levels is the same. The range of applicability includes sideline noise-levels for turbo-fan and turbo-jet powered aircraft with either rear fuselage or underwing mounted engines when the elevation angle of the aircraft@ relative to an observer at the sideline location@ is greater than five degrees@ the wind speed is less than 6 m/s (20 ft/s) and there are no temperature inversions. The use of the graphs and program is explained by means of a worked example that also illustrates the format of the input and output. An addendum discusses the estimation of sideline noise in subjective noise metrics.