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Key Words acoustics, ground effect, refraction, diffraction, scattering
Abstract Some applications of the study of outdoor acoustics and sets of data for sound-level spectra obtained close to the ground are described. Measurements and models of ground effects arising from the interaction between sound traveling directly from source to receiver and sound reflected from the ground are emphasized. Details are given concerning the influences of porosity, layering, small-scale surface roughness, and tall vegetation. Areas of related current and future research are outlined.
1. INTRODUCTION
1.1. Applications of Outdoor Acoustics
Sound in the atmosphere is a pressure wave. A single-frequency sound wave in air consists of a series of compressions and rarefactions that alternate in time and space around mean atmospheric pressure. Sound at low frequencies (< 100 Hz) can travel for considerable distances outdoors. Among the first experiments conducted on outdoor sound were those concerned with the speed of sound (Hunt 1992). The Franciscan friar Marin Mersenne (1588-1648) suggested timing the interval between seeing the flash and hearing the report of guns fired at a known distance. William Derham (1657-1735), the Rector of a small church near London, observed and recorded the influence of wind and temperature on sound speed. Derham also noted the difference in the sound, measured at the same distance, of church bells over newly fallen snow versus over a frozen surface. Before enough was known of outdoor acoustics for the military to exploit its use, there were many unwitting influences of propagation conditions on the course of battle (Ross 2000). In June 1666, Samuel Pepys noted that the sounds of a naval engagement between the British and Dutch fleets were heard clearly at some spots but not at others a similar distance away or closer (Naramoto 2000). The effects of the atmosphere on battle sounds were not studied in a scientific way until after World War I. During that war, acoustic shadow zones, similar to those observed by Pepys, were observed during the battle of Antwerp. Observers also noted that battle sounds from France reached England only during the summer months, whereas during the winter they were best heard in Germany. After the war, there was great interest in these observations among the scientific community. Large amounts of ammunition were...