There is a standard file structure for recording noise spectra and other data which is used throughout the ESDU Aircraft Noise Series. Most Items 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. 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 absorbtion attenuation or duct liner attenuation have to be made@ arithmetical combinination of spectral levels is required. A computer program known as ESDUpac A0818 accompanies this Item.