Abstract:

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.