ABSTRACT
Inmanyoutdoor noise prediction schemes, ground surfaces are considered as either ‘acoustically hard’, which means that they are perfectly reflecting, or ‘acoustically soft’, which implies that they are perfectly absorbing. According to ISO 9613-2 [1], any ground surface of low porosity is to be considered acoustically hard and any grass-, tree-, or potentially vegetation-covered ground is to be considered acoustically soft. Although this might be an adequate representation in some circumstances, it oversimplifies a considerable range of properties and resulting effects. Even the category of ground known as ‘grassland’ involves a wide range of ground effects. Where the main objective is to predict outdoor sound at long range including effects of discontinuous ground, meteorological effects, diffraction by natural and artificial barriers and topography, it is sensible to use the simplest possible descriptions of the acoustical properties of ground surfaces. However, in relatively simple propagation conditions, for example over flat grassland around airports, better accuracy should result from attempts to characterize the ground more completely. The interaction of sound with the ground includes several phenomena known collectively as ground effect. A convenient indicator of this interaction is the spectrum of the ratio of the total sound level at a receiver to the direct sound that would be present in the absence of the ground surface. This has been called the excess attenuation due to the ground surface and has been described in Chapter 2. As long as the ground may be considered as locally reacting, that is the surface impedance is independent of the angle of incident sound, the excess attenuation at a given receiver may be calculated from knowledge of the surface impedance and the source-receiver geometry. Most naturally occurring outdoor surfaces are porous. As a result of being able to penetrate the porous surface, ground-reflected sound is subject to a change in phase as well as having some of its energy converted into heat. The acoustical properties of porous ground are affected by the ease with which air can move in and out of the ground.
