**2. In situ tests for evaluating intrinsic characteristics of noise barriers**

A substantial part of the scientific literature on the evaluation of the acoustic properties of noise barriers has been based on the *in situ* evaluation of their intrinsic characteristics.

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

**Figure 2.** *Measurement according to the "Adrienne" method (left) [30] and "QUIESST" method (right) [31].*

In Europe, *in situ* tests for measuring intrinsic characteristics have been based on standardized methods such as AFNOR 31089 [29], the European projects "Adrienne" [30] and "QUIESST" [31], and more recently the EN 1793-4, 5 and 6 standards [32–34].

The measurement system consists of a fixed source (loudspeaker) reproducing a maximum length sequence (MLS) signal [30, 31] or a gunshot [29]. With these kinds of signals, the impulse response of an acoustic system can be obtained. In addition, background noise is eliminated [35]. Then a microphone is located behind the barrier to measure noise transmission or/and in front of the screen to measure noise reflection (**Figure 2**).

These methods are focused on the measurement at the near field (placing the microphones close to the surface to be measured) since, according to the standards, the lower power of the waves reflected, the difficulty of discerning between emitted and reflected sound, and the influence of background noise, make it really difficult to obtain meaningful results more distance [30, 31]. For this reason, some researchers prefer to extrapolate reflectivity data measured in the near field toward the effect in the far-field.

However, other researchers based on the standard EN 1793-4 [32], where receiver microphones are placed 2 m behind the barrier (see **Figure 3**), have situated the receiver microphones at greater distances from the screen (from 10 to 40 m) [36] considering these distances better to estimate the real IL.

#### **Figure 3.**

*Standard 1793-4 microphone location (based on [32]).*

In addition, according to Kim et al. [36], since the European test methods are based on an impulsive signal, they could be eliminating the influences of reflected sounds by ground and any other objects around the test area. So that, methods such as conventional Japanese and Kim et al. use traffic signal for their measurements.
