Analysis of environmental and seasonal effects on sound absorption by green wall systems

Buildings and roads are made of acoustically rigid materials which reflect sound emitted from many sound sources and contribute strongly to acoustic environment. In that context, green systems (roofs and walls) used for building envelope greening were found to be efficient solutions for acoustic protection in cities when applied in a significant urban scale. Green wall systems are usually composed of plants, soils and/or air gaps. Below 1000 Hz, sound attenuation is high in soils but much lower in plants and air. Thus, sound absorption by green systems varies with the arrangement of plants, air and soil and an efficient broadband absorption is only obtained when the arrangement is optimized. In this work, environmental (moisture content) and seasonal (leaves fall) effects on absorption coefficient are studied for a green wall system. Effective properties of plants and soils are measured in an impedance tube using three-microphones-two loads method between 100 and 1000 Hz under controlled moisture content for soils and porosity for plants. Sound absorption coefficient at normal incidence is then calculated using matrix manipulation. Variation of absorption coefficient with moisture and porosity is provided for a green wall system composed of Japanese spindle and coconut peat.