Environmental noise reduction with natural (soil and vegetation) barriers

Noise barriers are being used in recent years with more frequency to solve problems of noise pollution, in particular traffic noise from highways, above all in urban areas. They are normally long continuous structures, often more than 5 m high, made of various materials. What is needed is a way to provide the benefits of the noise reduction, creating at the same time a positive visual image for road users and the surrounding communities. The insertion loss of different types of natural barriers (an earth mound and a green-wall), constructed on Milan highways network were investigated, to ascertain the role played by the vegetation to increase the noise attenuation, in comparison with a completely artificial panel barrier (made of concrete and plexiglass). The ISO 10847 standard (In-situ determination of insertion loss of outdoor noise barriers of all types) was applied to carry out the measurements, being it suitable to evaluate the noise insertion loss of every type of acoustic barrier, as previously existing or builted in-situ. The indirect method, to be followed when the barrier has been already installed and cannot be removed, was applied: the insertion loss is evaluated by comparing the noise level in different measurement points, at the same distance from the source and for equal (or at least similar) environmental noise propagation, being some locations placed behind the barrier, and others defined in a way to completely exclude the barrier influence. For the earth mound covered by vegetation, an average difference of 2.4 dB(A) between the levels recorded in winter (no leaves) and in summertime (full growth) was obtained, substantially confirming what drawn out from previous similar studies. Nevertheless, the vegetation contribution to noise reduction is remarkably less significant in comparison with the performance of the earth mound with no vegetation (insertion loss of about 10 dB(A)) and also that of the artificial panel (about 25 dB(A)). The frequency analysis shows a significant contribution of the leaves in reducing the noise levels in the medium (500 and 1000 Hz) frequency bands. Unfortunately, the traffic vehicle produces typically peaks in 63 and 125 Hz bands, so the vegetation is not so efficent in this case. Nevertheless, the human hearing shows its maximum sensitivity in the range from 500 to 2000-3000 Hz, so the vegetation improves the situation. However, it should be taken into account that the vegetation contribution of a barrier is not only represented by the noise attenuation, but also it acts as a sort of filter, able to reduce the air pollution in the area immediately behind the barrier itself, being at the same time a very powerful factor to improve, sometime dramatically, the urban scenery. The possibility to build a natural barrier assumes in this case a strategic importance in a view of a correct environmental management