Modeling of sound propagation in forests using the Transmission Line Matrix method: Study of multiple scattering and ground effects related to forests

Abstract : The prediction of sound propagation in presence of forest remains a major challenge for the outdoor sound propagation community. The investigation of forest as sound propagation medium is motivated by the propensity of a forest to mitigate noise, the protection of existing "green" quite area (Environmental Noise Directive 2002/49/EC) and the preservation of biodiversity. Absorption from forest ground at low frequencies, multiple scattering caused by tree trunks and the modification of temperature and wind gradients are the three main phenomena that directly impact sound propagation in significant forested areas. Reference numerical models such as the Transmission Line Matrix (TLM) method can be developed in order to accurately predict each acoustical phenomenon that takes place inside forest. This time domain method has already been used to model long range sound propagation and is well suited for the prediction of transient phenomena. The first need for the TLM method is an efficient theory-based absorbing layer formulation that enables the truncation of the numerical domain. This aims at lowering the computational cost of the method. The two proposed absorbing layer formulations are based on the approximation of the perfectly matched layer theory. The most efficient proposed formulation is shown to be equivalent to wave propagation in a lossy media, which, in the TLM method formulation, is introduced using an additional dissipation term. In comparison to the existing empirical one-way approach, the proposed absorbing matched layer, after optimization, presents lower reflection errors for large angle span. The ability of the TLM method for the simulation of scattering is studied comparing the numerical results to both analytical solutions and measurements on scale models. It is shown that both single and multiple scattering predicted with the TLM method are in good agreement with the theory. Moreover, the measurements of scattering above either reflecting or impedance grounds show that the TLM method predictions are in correct agreement with the measured data. Lastly, the attenuation of acoustic levels by a simplified forest is numerically studied using several arrangements of cylinders placed normal to either reflecting or absorbing ground. It is observed that randomly spaced arrangements are more inclined to attenuate acoustic waves than periodic arrangements. Moreover, the sensitivity to the density, the length of the array and the ground absorption is tested. The main trend shows that the density and the distribution are two important parameters for the attenuation. In future work, it can be interesting to look at the sensitivity of each parameter. This study could then be used to relate the morphology (i.e. distribution, density, length) of a forest to the acoustical properties of the forest.
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Pierre Chobeau. Modeling of sound propagation in forests using the Transmission Line Matrix method: Study of multiple scattering and ground effects related to forests. Acoustics [physics.class-ph]. Université du Maine, 2014. English. ⟨tel-01137915⟩

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