The effect of small changes in source height on the propagation of sound over grassland

Abstract

In this paper the results of measurements of the propagation of sound from a jet engine source at three different heights are given. Two series of measurements were made with the source first at 3 ft and then at 15 ft above the ground. Propagation measurements were made under a wide range of wind conditions for both source heights and all the measurements were confined to periods of temperature lapse. The propagations from these two source heights are compared, together with results from previous measurements over the same site at Hatfield with the source 6 ft above the ground. For the measurements at all three source heights the microphones were kept at 5 ft above the ground, spaced at logarithmic intervals from the source along a line 3600 ft long. As was expected from a previous pilot experiment and from theoretical considerations, the frequency of the maximum ground effect decreased with increasing height, and wind-created shadow formation took place more readily with the source at the lower height. The effects of the small changes in source height on the sound levels received at the microphones varied with propagation conditions. Under conditions favourable for propagation the average reduction in received level due to reducing the source height from 15 ft to 3 ft was about 3 dBA. Under neutral and slightly unfavourable propagation conditions the corresponding reduction in received levels was about 5 dBA and similar reductions were found for the closer microphones under unfavourable propagation conditions. At the more remote microphones under unfavourable conditions, small increases in level, about 1 dBA, were found due to the reduction in source height. The results from the source at 6 ft above the ground in general carne between those from 3 ft and 15 ft above the ground. However, this was not the case at the remote microphones under conditions favourable for propagation, where the levels from the source at 6 ft high were higher than those from either of the other two heights. The reason for this anomaly is not known. It is concluded that for broad band noise propagated over grassland, a significant reduction in sound levels received at a distance can be achieved by lowering the source height from 15 ft to 3 ft.