6.2 Ground effects and terrain modeling

Ground effects and terrain modeling play a crucial role in outdoor sound propagation. These factors influence how sound waves interact with the earth's surface, affecting reflection, absorption, and overall sound levels at different distances from the source.

Understanding ground impedance models is key to predicting sound behavior across various terrains. From simple empirical models to advanced approaches considering porosity and pore structure, these tools help engineers accurately estimate sound levels in outdoor environments, accounting for hills, valleys, and barriers.

Ground Reflection and Absorption

Principles of Ground Reflection and Absorption

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• Sound waves propagating outdoors interact with the ground surface, resulting in reflection and absorption of sound energy
• The amount of reflection and absorption depends on the acoustic properties of the ground surface, characterized by its acoustic impedance
• Hard surfaces (concrete, water) have high acoustic impedance and reflect most of the sound energy
• Soft surfaces (grass, snow) have low acoustic impedance and absorb more sound energy
• The reflection of sound from the ground can lead to constructive or destructive interference with the direct sound, affecting the overall sound level at the receiver
• Ground absorption is frequency-dependent, with higher frequencies typically being absorbed more than lower frequencies

Effects of Ground Reflection and Absorption on Sound Propagation

• The combined effects of ground reflection and absorption can result in sound level variations as a function of distance from the source and the height of the source and receiver above the ground
• At short distances, the direct sound and reflected sound can interfere constructively, leading to an increase in sound levels
• As the distance increases, the path length difference between the direct and reflected sound increases, resulting in destructive interference and a reduction in sound levels
• The presence of ground absorption can further modify the sound levels, particularly at high frequencies, leading to additional attenuation
• The height of the source and receiver above the ground influences the extent of ground reflection and interference effects
• Elevating the source or receiver can reduce the influence of ground reflections and minimize interference effects

Ground Impedance Models for Terrain

Empirical Ground Impedance Models

• The Delany-Bazley model is a widely used empirical model that relates the flow resistivity of a porous ground surface to its acoustic impedance as a function of frequency
• The model is based on measurements of various porous materials and provides a simple way to estimate the acoustic impedance of outdoor ground surfaces
• The Miki model is an improved version of the Delany-Bazley model, providing more accurate predictions for a wider range of flow resistivity values
• The Miki model addresses some of the limitations of the Delany-Bazley model, particularly at low frequencies and for surfaces with high flow resistivity

Advanced Ground Impedance Models

• The variable porosity model takes into account the depth-dependent porosity of the ground, allowing for more accurate modeling of layered soil structures
• This model considers the variation of porosity with depth, which can significantly influence the acoustic properties of the ground
• The Zwikker-Kosten model is suitable for modeling the acoustic properties of rigid-framed porous materials, such as asphalt or concrete
• The model accounts for the complex pore structure and the viscous and thermal losses within the porous material
• The selection of an appropriate ground impedance model depends on the specific terrain type, frequency range of interest, and available input parameters
• More advanced models, such as the Biot-Allard model or the Wilson model, can provide more accurate predictions but require additional input parameters and computational resources

Application of Ground Impedance Models

• By incorporating the ground impedance model into outdoor sound propagation calculations, the effects of ground reflection and absorption can be accurately predicted for various terrain types
• Ground impedance models are used in conjunction with sound propagation algorithms, such as the ISO 9613-2 or Nord2000 models, to estimate sound levels at different distances from the source
• The accuracy of the predictions depends on the appropriateness of the selected ground impedance model and the quality of the input data, such as flow resistivity measurements or soil properties
• Validation studies have shown that ground impedance models can provide reliable predictions of outdoor sound levels, particularly when combined with accurate source and meteorological data

Terrain Effects on Sound Propagation

Influence of Hills and Berms

• Hills and berms can act as natural barriers, diffracting and attenuating sound waves that pass over them, resulting in reduced sound levels behind the obstacle
• The attenuation provided by a hill or berm depends on its height, shape, and the frequency content of the sound source
• Higher hills or berms provide greater attenuation, as they create a larger path length difference between the direct and diffracted sound waves
• The shape of the hill or berm, such as its slope and curvature, can influence the extent of diffraction and the resulting sound levels behind the obstacle
• The frequency content of the sound source affects the diffraction efficiency, with lower frequencies diffracting more easily around obstacles compared to higher frequencies

Sound Focusing in Valleys

• Valleys can focus sound energy, leading to increased sound levels at certain locations due to the convergence of reflected sound waves
• The shape and orientation of a valley relative to the sound source and receiver can affect the extent of sound focusing and the resulting sound levels
• Concave valleys can act as acoustic mirrors, reflecting sound waves towards a focal point and creating localized areas of high sound levels
• The focusing effect depends on the geometry of the valley, the reflective properties of the valley walls, and the position of the source and receiver
• In some cases, the focusing of sound in valleys can lead to sound levels that exceed those in open terrain, particularly at low frequencies

Modeling Terrain Effects

• Modeling the effects of terrain features on sound propagation requires the use of advanced numerical methods, such as the Parabolic Equation (PE) or the Fast Field Program (FFP)
• These methods can account for the complex geometry and acoustic properties of the terrain, including the shape and composition of hills, valleys, and other features
• PE and FFP models solve the wave equation in a two-dimensional or three-dimensional space, taking into account the variations in terrain height and ground impedance
• The models can predict sound levels at different locations in the presence of terrain features, considering the combined effects of diffraction, reflection, and absorption
• The accuracy of the predictions depends on the resolution of the terrain data, the appropriateness of the boundary conditions, and the computational resources available

Noise Reduction by Barriers

Natural Barriers

• Natural barriers, such as hills, berms, or dense vegetation, can provide noise reduction through a combination of diffraction, absorption, and scattering of sound waves
• The effectiveness of a natural barrier depends on its height, width, and the materials it is composed of, as well as the frequency content of the noise source
• Hills and berms can act as effective barriers, providing significant noise reduction behind the obstacle, particularly at high frequencies
• Dense vegetation, such as trees or shrubs, can provide some noise reduction through scattering and absorption of sound waves, but their effectiveness is limited compared to solid barriers
• The noise reduction provided by vegetation depends on factors such as the density, height, and width of the vegetation, as well as the leaf size and shape

Artificial Barriers

• Artificial barriers, such as walls or fences, can be constructed from various materials (concrete, masonry, wood, metal) to block the direct path of sound waves
• The noise reduction provided by an artificial barrier is determined by its height, length, and the sound absorption properties of the barrier material
• Higher barriers provide greater noise reduction, as they create a larger path length difference between the direct and diffracted sound waves
• The length of the barrier should be sufficient to prevent sound from diffracting around the edges and reaching the receiver
• Sound-absorbing materials, such as porous concrete or mineral wool, can be used on the barrier surface to minimize reflections and increase the overall noise reduction
• The design of an effective noise barrier should consider factors such as the location and height of the barrier relative to the source and receiver, the barrier material and surface treatment, and the presence of any gaps or openings

Comprehensive Noise Control

• The use of noise barriers in combination with other noise control measures, such as source modification or receiver insulation, can provide a comprehensive approach to reducing outdoor noise levels
• Source modification techniques, such as enclosures, silencers, or low-noise equipment, can reduce the noise emission at the source itself
• Receiver insulation, such as sound-insulated windows or façade treatments, can reduce the noise levels inside buildings or sensitive areas
• The selection and design of noise control measures should be based on a thorough assessment of the noise source characteristics, the propagation path, and the receiver requirements
• Numerical modeling and on-site measurements can help optimize the design and placement of noise barriers and other control measures for maximum effectiveness
• Regular maintenance and monitoring of noise control measures are essential to ensure their long-term performance and effectiveness in reducing outdoor noise levels