INTRODUCTION There is a standard file structure for recording noise spectra and other data which is used throughout the ESDU Aircraft Noise Series. Most Items@ but not yet all@ employ this Common Data standard. There is an on-going programme to enable every relevant program to read and write such data files. This standard file specification enables computer programs from the ESDU Aircraft Noise Series to be run successively@ that is@ the output from one program can be used as an input for another. For example@ the user can estimate the noise of a single-stream jet engine in flight for a range of angles and distances by running first the far-field@ single-stream@ static jet noise prediction program and using the output in the Common Data File generated by this program as the input to the static-to-flight effects program. At other times@ the user may wish to combine spectral levels contained in Common Data Files that have been generated by different programs. This Item facilitates the combination of spectral levels. Spectra at any given location can be added or subtracted either on a logarithmic or an arithmetic basis. Aircraft or engine noise is usually predicted by calculating the noise produced from more than one source and then adding the individual source noise predictions together. The addition of such spectra must be performed on an energy@ i.e. logarithmic@ basis. If@ however@ corrections like those for ground-reflection@ atmospheric absorption attenuation or duct liner attenuation have to be made@ arithmetical combination of spectral levels is required. This Item allows the user to add or subtract spectral levels at the particular locations contained in the Common Data Files either arithmetically or logarithmically. It is important to check the appropriate ESDU Item for the polarity of the calculated correction to determine whether the correction needs to be added or subtracted. For example@ if a free-field spectrum is to be corrected for ground-reflection@ the correction from ESDU 94035 will have to be arithmetically added. To correct a measured spectrum to free-field condtions@ the ground-reflection correction will have to be subtracted. Figure 2.1 illustrates how this Item and other ESDU Aircraft Noise Items may be used to estimate the flyover noise of an aircraft with two turbofan engines with fan inlet duct liners and a fully-mixed exhaust system@ at a reception point close to the the ground@ after propagation through a real@ still atmosphere.