ESDU 89036:2012

ESDU 89036 is accompanied by a computer program, ESDUpac A8936,
which calculates a difference noise spectrum in one-third octave
bands which, when subtracted from a measured spectrum, will yield
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.