IHS ESDU The correction of measured noise spectra for the effects of ground reflection. 94035

ESDU 94035 introduces a Fortran program to calculate a difference spectrum between the measured and free-field spectra either at discrete frequency noise levels at constant intervals or at one-third band levels of broad-band noise. The method takes into account both ground impedance through a two-parameter model and atmospheric turbulence by considering its effect on the coherence between the direct and reflected rays, and guidance is included on values for the parameters required to define the impedance model and the turbulence level. The program may also be used to correct a measured spectrum, automatically adjusting for any discrepancy between the cancellation frequencies of the predicted and measured spectra arising from such effects as atmospheric turbulence, errors in source location, or timing errors. An allowance may be made for the characteristics of the filters used to analyse measured data. The method applies to a discrete, stationary source but techniques for taking account of source velocity and distributed source effects are discussed. The minimisation of reflection effects by suitable choice of microphone height is considered. The input to and output from the program are illustrated by a worked example and the programs, ESDUpac A9435 and ESDUpac B9435, are provided.