ABSTRACT
Outdoor sound is influenced by distance, by topography, by interaction with the ground and with or without vegetative ground cover and by atmospheric effects including refraction and absorption. The total attenuation of a sound outdoors can be expressed as the sum of the reductions due to geometric spreading, atmospheric absorption and the extra attenuations due to ground effects, scattering, visco-thermal effects in vegetation, refraction in the atmosphere and diffraction by barriers. The extent to which discontinuities in surface impedance can influence the rate of attenuation is illustrated by data from measurements of noise levels during aircraft engine run-ups made at distances of up to 3 km from the source with the aim of defining noise contours in the vicinity of airports. Outdoor sound prediction requires information on wind speed, direction, temperature, relative humidity and barometric pressure as a function of height near to the propagation path.
