Acoustical Properties of Farmland

Although many measurements have been made of the propagation of sound over grassland, there are relatively few data over cultivated surfaces. The propagation of sound over an absorbing surface is influenced by the impedance of the surface and the locations of the source and the receiver. A reduction in ground impedance can increase the rate of attenuation of Aweighted sound levels due to broad-band sources near to the ground. Measurements of complex excess attenuation at 1 m from a point source have been used to deduce the acoustical impedance of a variety of farmland surfaces including arable land. These data suggest that farmland surfaces have lower impedance values than those assumed typical of grass-covered surfaces in noise prediction schemes. Spectra of the difference in levels measured at vertically separated microphones at distances of 30 and 44 m from a loudspeaker over four cultivated surfaces are presented. The differences between spectra measured over subsoiled or ploughed ground and those obtained over relatively undisturbed surfaces are consistent with predictions that take into account the effective impedance of rough porous surfaces. The rates of attenuation of A-weighted broad-band sound over an oilseed rape crop, and over ploughed and undisturbed portions of a wheat stubble field at ranges up to 50 m are found to be greater than predicted by an empirical ISO method. A simple semi-empirical model that uses a single ground parameter is found to give better agreement with these data.