Determination of the impedance of vegetated roofs with a double-layer Miki model

Vegetated roof systems on the top of buildings can act as absorbers for traffic noise mitigation. Recent research explains the properties of vegetated roofs that are important for their sound absorptive and scattering properties. Although it has been identified that the substratum is the major contributor to the acoustic absorption of a vegetated roof, coverage of this substratum by plants may have a significant effect on the acoustic absorption. Short-range acoustic propagation experiments have previously been used for in-situ measurements to determine the acoustic impedance of surfaces as forest floors, grasslands, and gravel. However, it is still less practical to estimate the impedance of a non-locally reacting layer of leaves on substratum using the proposed method. The Miki model provides a satisfactory prediction of the fundamental acoustic properties of soils, plants, and their combinations with the advantage of computational simplicity. Here, the double-layer Miki model is examined based on the short-range acoustic propagation method over substratum (with and without layer of leaves) in the laboratory, and considered through in-situ outdoor measurements over green roofs. In addition, the application of the double-layer Miki model on the prediction of the non-locally reacting surface impedance is evaluated by a comparison with other impedance models.