*3.1.4 Main findings*

There is a lack of evidence on the effectiveness of barriers based on the direct method. In the study conducted by Anfosso-Lédée et al. [40], the authors suggest that it could be due to the poor applicability of the method since it took 3 years for his team to complete the measurements due to the time waited for the installation of the barrier and the difficulty to find the equivalence of traffic, weather conditions, and ground impedance.

The experiment was based on measurements at 30 m and 100 m from the barrier (or where the barrier was supposed to be installed) (**Figure 5**). The measurements "before" took place in 1996, and the measurements "after" (when the barrier was already installed) were implemented from June 1998 to August 1999. Results showed IL values range from 4 to 8 dB(A) at 100 and 30 m from the road.

Parnell et al. [41] constructed a new barrier 80 m long and 2.4 m high and measured, with and without the barrier, at a distance of 2.4 m in front of the barrier and 2.4 and 4.8 m behind the barrier (**Figure 5**). Results of the experiment showed a 6–8 dB(A) difference in measurements "before" and "after" the barrier installation.

## **3.2 Indirect measurement method**

The indirect method is, according to ISO 10847:1997, the approach to be used when the noise barrier has already been installed and cannot be removed for measurements.

*Approaches for Noise Barrier Effectiveness Evaluation Based on* In Situ *"Insertion Loss"… DOI: http://dx.doi.org/10.5772/intechopen.104397*

In this case, an estimated "before" noise level is obtained by the measurement at a site that is considered equivalent to the study site. To ensure consistency of results, the "before" and "after" measurements should be performed simultaneously.

The indirect method is the only practicable approach in the case of most new roads, where the noise barriers have been installed during road construction, and therefore it is not possible to obtain a "before" measurement under normal traffic conditions. The primary advantage of using this method is that it ensures the same environmental conditions (meteorological and traffic conditions), so this method, as highlighted by some authors [17, 42], would be preferred over the direct measurement method.

The use of the indirect method involves the identification of another measurement site that is deemed to be equivalent. For these equivalent sites, a close match is required in emission characteristics, relative positions of source, barrier and receiver, acoustic performance of ground surface, terrain profile, interfering obstacles, reflecting surfaces, and meteorological conditions. The factors to be considered in the determination of the measurement sites and procedures are briefly described below.

#### *3.2.1 Selection of equivalent sites*

According to ISO 10847:1997, the "before" site must have a terrain profile, interfering obstacles, and reflecting surfaces equivalent to those of the real barrier site within a sector extending 60° on either side of the line connecting the receiver positions towards the source position, so that similar noise propagation can be achieved.

It is also necessary to ensure the equivalence of ground surface, which refers to the acoustic impedance of the ground along the source-receiver propagation path (i.e., acoustic characteristics of soil coverage, such as paved soil, vegetation on loose or packed soil, gravel, etc.) (**Figure 6**). The standard ISO additionally requires that

#### **Figure 6.**

*An example of "after" and equivalent "before" locations at one of the sites studied by the authors in Spain (aerial photograph from Iberpix, OrtoPNOA 2020 CC-BY 4.0 scne.es).*

the environment in the region within 30 m behind and to the side of the receiver positions shall be similar.

The main difficulty of the method is that an adjacent equivalent site may not always be available, especially in dense urban areas [17, 40]. As an example, a study conducted in Spain [42], which was based on an initial sample of 84 measurement sites, had to reject 54 potential locations due to various causes; the main cause was the different acoustic environment at the "before" and "after" positions due to significant differences in terrain profile and the presence of other noise sources.
