**6. Results and discussion**

The values of SPL at P2 obtained by measurement and by propagation from the source with two different depletion laws showed that the best fitting was obtained when considering a linear depletion. We focused our comparison on frequencies up to 500 Hz, because at upper frequencies the differences could be related to other phenomena.

When using an approach similar to ISO 9613-2 to propagate the SPL from the source to P1 without considering the eucalyptus plantation, a 12 dB difference was found. This difference could be due to the presence of the trees, which could behave as an acoustic barrier. The greatest extra attenuation was obtained at the frequencies of 1000 Hz and 2000 Hz; according to Tunick [17], this is the frequency range where the trunks, branches, and crowns have their best acoustic performance. Some extra attenuation was also found below 500 Hz; according to Martínez-Sala [18], the differences in this range would be attributed to the destructive interference of the sound waves when scattered in a belt of trees planted following a periodic pattern.

For answering the question about which are the best equations to predict the behavior of the vegetal barrier, the following results are discussed.

The best performance was expected for those formulae developed for green barriers, as Hoover's or the ISO 9613-2 correction term for green barriers. But when calculations were done, they achieved the worst results, being ISO 9613-2 worse than Hoover's (see **Figure 13**).

Just the opposite, the best result for the green barrier IL was achieved in the thick barrier approach, and the second in accuracy was the Kurze-Anderson approach.

In both cases, when adding the edge diffraction, the results did not exhibit any changes, i.e., the edge diffraction was significantly lower than the upper one.

It is noticeable that the results at the frequencies where green barriers are expected to have better performance (1000 Hz and 2000 Hz, according to [3] and [17]) are particularly accurate in both cases. Since these methods have not been developed for green barriers, we did not expect these results.

In order to explain these results, we think that it is possible that the sound waves could behave as if the barrier was a solid obstacle, regarding the long distance between the source and the receiver. Since our atmospheric measurement conditions according to Pasquill-Gifford (see, e.g., [29]) were unstable or neutral atmosphere (wind velocities lower than 5 m/s, variable insolation conditions), this interpretation could oppose [20], by assigning no importance to atmospheric stability conditions.
