REFLECTION AND ABSORPTION OF SOUND
These topics are addressed below:
Reflection and Absorption of Sound from Walls
Absorption by Flat Ground Surfaces
Reflection and Absorption of Sound from Walls
When a sound wave strikes a surface, it is reflected. However, some of the wave may be absorbed. The absorption coefficient may be read from the Table knowing the dB reduction that occurred on reflection.
For most materials, the average absorption of a surface is reasonably well defined by an angle of incidence of 45°.
dB Change Absorption Coefficient
Absorption by Flat Ground Surfaces
The term "ground interference effect" is used to describe the broad absorption effect of ground such as sand, earth and fields. Hard surfaces such as roads and parking lots show an interference effect too, as shown by a blue line below, but are not included in the term "ground interference effect".
The ground interference effect provides natural sound attenuation as the wave grazes across open land. In most cases of sound propagation outdoors, the ground surface does not supply much sound absorption in comparison with the perfect absorption provided by the sky. However, the impedance of the ground is important in providing sound attenuation through the so-called ground interference effect.
The figure above shows the basis for this effect -- when the ground is soft, the wave that reflects can he out of phase with the direct wave from source to receiver. This leads to destructive interference over a broad frequency band. The upper limit of this frequency band is determined only by geometry, and automatically scales if the full scale geometry is reproduced in the model.
The lower limit of this band, and the magnitude of the interference effect are determined by geometry and impedance of the ground. The data in the two figures show the simulation of ground effect by two different research groups.
In both cases, the field data is that of Parkin and Scholes.
