TENTATIVE METHOD FOR CALCULATION OF THE SOUND FIELD ABOUT A SOURCE OVER GROUND CONSIDERING DIFFRACTION AND SCATTERING INTO SHADOW ZONES

摘要

A semiempirical method is given for the calculation of the Bound field about a source over ground considering the effects of vertical temperature and wind gradients as well as scattering of sound by turbulence into shadow zones. The diffracted field in a wind-created shadow zone is analyzed theoretically in the two-dimensional case and is shown to be similar to the results obtained for a temperature-created shadow field as given in NACA TN 3494. The frequency and wind-velocity dependence of the scattered field into the shadow zone is estimated from Lighthill's theory, and on the basis of these two field contributions a semiempirical formula is constructed for the total field which contains two adjustable parameters. From this expression a set of charts has been prepared showing equal sound-pressure contours at 10 feet above ground for various source heights, wind velocities, and frequencies.