*5.4.1 North side diffraction*

**Figure 12** presents the diagram to be used for calculating the North side diffraction. When Kurze-Anderson or the thick barrier approach is used, the calculated sound pressure levels at the receiver were lower than the measured background noise. Thus, they are negligible.

*Evaluation of Industrial Noise Reduction Achieved with a Green Barrier: Case Study DOI: http://dx.doi.org/10.5772/intechopen.108835*

### *5.4.2 South side diffraction*

**Figure 11** shows that before the sound reaches the South side of the barrier, it must pass through another plantation of trees along 110 m. After reaching the barrier under study, it has to pass through another forestation, in order to reach the receiver. This complex path may impose greater attenuation than a single tree barrier. Even if there are no simplified methods for calculating the SPL at the receiver in this case, the low SPL obtained at the North side allows to expect negligible results at the receiver.

#### **5.5 Comparison of the results achieved by different calculation methods**

**Figure 13** shows the SPL in octave bands obtained for different situations and calculation methods.

The blue lines represent the SPL measured at P1: the bottom dotted line corresponds to the background SPL, and the solid one represents the SPL measured when the coal processing mill was the only operating source at the industrial plant.

The upper dotted line represents the calculated SPL without considering the tree barrier (Ldir).

All the other lines correspond to different calculation approaches. The green ones were obtained by calculation with methods that consider green barriers (Hoover's and Afol from ISO 9613-2); the red and orange ones correspond to the calculation methods for solid barriers.

**Figure 13.** *Comparison of results at P1.*
