Effect of tree species and season on the ability of forest floors to abate environmental noise

Abstract

The nature of the ground beneath a belt of trees along transport infrastructure can contribute significantly to the associated noise abatement. To get more insight in this topic, repeated measurements were performed, both in winter and summer time, in a common garden of six monoculture broadleaved tree stands (ash, cherry, lime, maple, beech and oak, all of the same age). A two-microphone technique was used to derive the ground surface impedance and soil properties describing the interaction between the forest floor and sound waves. The slit-pore and Zwikker and Kosten (phenomenological) ground impedance model, both in rigid backing configuration, were found to adequately predict the measured short-range level differences. The much higher moisture content of the forest floor during the winter campaign resulted in a strong increase in the deduced surface impedance. A clear overall tree species effect seemed absent, despite large differences in litter degradation rates. Additional measurements after raking away the organic layer allowed concluding that a pronounced forest floor might compensate for an acoustically harder mineral soil beneath. In tree belts aiming at noise reduction, litter should not be removed and care is needed not to compact the mineral soil, especially in case of stands with rapidly decaying litter and a thin organic horizon.