Investigation on the Effect of Moisture Content in Mixed Substrates of Green Walls on Sound Diffraction

The substrates used in living walls are basically porous media, supplying air and water to plant roots. While previous studies have explored the sound absorption properties of green walls, the quantitative impact of moisture content in substrates remains unverified. This study investigates how moisture in mixed substrates (perlite, coco peat and coco fiber) influences diffracted sound waves. First, the acoustic properties of mixed substrates were characterized through impedance tube measurements, revealing that moisture content significantly affects their performance. In the second part, these results were incorporated into a numerical model using the Boundary Element Method (BEM) to assess the effectiveness of green wall substrates in mitigating diffraction and transmission of noise. The model predicts noise reduction at six receiver points, positioned 5 and 10 meters from the sound source. Findings indicate that dry substrates effectively reduce low-frequency noise, particularly around 150 Hz. In urban environment, lower frequencies noises are more likely to exhibit diffraction effects, therefore it is important to understand the effects of substrates and properly maintain the water content. This shows green walls containing substrates not only exhibit good sound absorbing performance but also suppress the diffracted sound on the surface of the wall.