Porous ground, crops, and buried resonators

Transport noise travelling near to naturally occurring porous ground surfaces, such as grassland, is reduced at frequencies relevant to noise control as the result of interference between direct and groundreflected sound known as the ground effect. Although it is observed frequently that different types of naturally occurring ground surfaces give rise to different ground effects, little thought has been given to deliberately choosing ground to give an improved noise reduction. In this chapter, results of calculations and measurements are provided that can inform this choice. Dense vegetation, in the form of ground cover or crops, contributes to ground effect as a result of the penetration of roots into soil and adds extra attenuation due to scattering by leaves and stems. Calculations are provided that suggest the potential usefulness for noise control of certain combinations of ground and crops. Experiments and calculations are reported showing that ground that is otherwise acoustically hard, such as nonporous asphalt or concrete, can be altered to be more effective for noise reduction by being made porous or by inserting porous strips or patches composed, for example, from gravel. Porous asphalt road surfaces are used to reduce both noise generation and propagation from road–tyre interaction. Their effectiveness for reducing traffic noise can be increased by burying preformed resonating chambers. Resonators also can be buried in hard or porous ground to reduce noise. Laboratory data and predictions are presented that demonstrate the usefulness of this method