Influence of transducer locations on acoustic impedance prediction in porous systems with application to vegetated roofs

Urban vegetation, such as vegetated roofs, is effective in mitigating urban noise. In previous research, the acoustic impedance of vegetated roofs could be predicted by minimising the differences between the measured and pre-calculated level differences between two vertically placed microphones using the multiple-geometry technique. However, it was found that for some cases, the predicted sound pressure level differences deviated from the measured ones at the interference peaks. The sound pressure level produced by a single sound source above a ground surface is characterized by the interference of the direct and ground reflected sound waves, and the accuracy of the assumed locations of the transducers influences the prediction of the ground surface impedance. Therefore, the sensitivity of the transducers’ locations on the determination of the acoustic impedance of porous materials using the multiple-geometry technique was assessed in this research. It was found that small errors on the transducers’ locations lead to significant variations in predicted impedance and material properties. The tolerance of the extracted transducers’ locations is recommended to be within ± 0.005 m to achieve an accurate and unique prediction of the surface impedance of porous materials.