ESDU 89036 introduces a Fortran program@ provided on disc (uncompiled) in the software volume as ESDUpac A8936@ that calculates a difference noise spectrum in one-third octave bands which@ when subtracted from a measured spectrum@ will yield the free-field values. The method assumes a linear sound speed gradient@ and has been validated against limited experimental data drawn from the literature for distances up to 120 m (400 ft) and for source and receiver heights up to 2 m (6.5 ft) in the frequency range 100 Hz to 5 kHz. The accuracy was within 3 dB. The method represents the acoustic impedance of the ground in terms of porosity and specific flow resistance@ and guidance is given on the values to be taken for those quantities. The theory is outlined@ and a worked example illustrates the form of the input data and the output results.