Foliage as a low-pass filter: Experiments with model forests in an anechoic chamber.

In order to investigate the influence of foliage on sound transmission through vegetation, we have examined four model forest situations in an anechoic chamber. In general, we found that the foliage acts in the midfrequencies as a noise amplifier, which is important for the vocalization and communication of animals. In the high frequencies the foliage of plants acts as a good noise filter. The acoustic properties of this filter are determined by the plant species, and depend mainly on the maximum size of the plant organs. A clear correlation is found between the maximum size of the plant leaves and the frequency, from which the filtering capacity of the plants starts. Also, a correlation could be detected between the total biomass of a specific plant species and the noise ’’attenuation’’ in the 1/3-octave bands that are examined. The total noise attenuation caused by the vegetation depends on the spectrum of the noise: the sound pressure level of a noise spectrum with high levels in the high frequencies will be decreased by the plant foliage but the traffic noise spectrum will be changed only in pitch. We conclude that this filtering property of plant communities could be useful in noise abatement.