PROPAGATION FROM A BROAD-BAND SOURCE OVER GRASSLAND: COMPARISON OF DATA AND MODELS

摘要

Good agreement with the data below 1000 Hz for zero wind conditions follows from use of equation (1) with the grassland characterized by effective flow resistivity 30 kPa s nr2 and effective depth 0.05 m and with turbulence characterized by outer scale 1.0 m and mean squared refractive index 10'7.The empirical scheme proposed for aircraft ground operations noise agrees fairly well with measured overall A-weighted levels beyond 500 m from the source but tends to overestimate attenuation bewteen 150 and 500 m from the source. Downwind levels are such that 10 dB less attenuation is measured at 1000 m from the source than during acoustically-neutral conditions. The degree of turbulence is important in whether or not downwind levels are greater than those obtained under acoustically-neutral conditions.The main ground effect dip may occur in an octave band below that predicted by the ISO scheme and attenuations in the 1000 Hz and 2000 Hz octave band may be significantly less in downwind conditions than predicted by the ISO scheme.