ESDU 94036 introduces a Fortran program for calculating either a difference spectrum between a source and a point close to the ground or@ knowing the spectrum at one point@ the spectrum at another point@ both at discrete frequency noise levels at constant intervals or at one-third octave band levels of broad-band noise. The effects of spherical spreading@ atmospheric attenuation@ and ground reflection may be treated separately or in any combination. For reflection a two-parameter ground impedance model is used and atmospheric turbulence is treated through consideration of its effect on the coherence between the direct and reflected rays. Guidance on quantities required to define the ground impedance model and the turbulence level is provided. The input to and the output from the program@ ESDUpac A9436@ are illustrated by a worked example@ and the program is provided on disc in the software volume both uncompiled and compiled in ESDUview@ a user-friendly shell that prompts for input data on screen.