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Description
The diffusion equation model has been used for room acoustics simulations because of its computational efficiency. Traditionally, the diffusion coefficient of the diffusion equation has been treated as a constant depending on the room's volume and total surface area. However, numerical investigations have demonstrated that in elongated spaces, this assumption does not hold: the diffusion coefficient varies with spatial location, source position, and absorption coefficient of the boundaries. Despite these theoretical insights, experimental validation of spatial dependencies is lacking. This study addresses this gap by presenting experimental measurements of the diffusion coefficient within one of the tunnels of the Schoenenbourg fort, a structure belonging to the historic Maginot line in France. The methodology relies on intensity and energy density measurements taken along the tunnel, allowing the diffusion coefficient to be derived directly from its definition given by Fick's law. The measurement results demonstrate that the diffusion coefficient cannot be regarded as a constant in elongated spaces but instead depends on spatial conditions. In addition, a preliminary validation was performed by incorporating the experimentally obtained diffusion coefficient into the diffusion equation model. The comparison revealed that the model, when used with the measured diffusion coefficient, successfully reproduces the tunnel sound pressure level and reverberation time.