# Possibilities of Evaluating the Effectiveness of Noise Barriers in Slovakia
## 3 Results
### Determination of the Insertion Loss of a Barrier by the Indirect Method  
To determine the input of noise barrier attenuation, the indirect measurement method according to [28] was chosen. The measured sound barrier is located in the section of the Motorway D1 Hricovske Podhradie-Lietavska Lucka in the direction from Bratislava to Poprad on the right side, between the villages of Lietavska Lucka and Bytcica at the end of the bridge on the bridge dilatation according to stationing km 35,416. Reception points represent the open space behind the barrier and are situated so that there are no obstacles in the sound propagation. Sound barrier SO 241-00 consists of polymethacrylate panels in an aluminum frame (PMMA) with a height of H = 4 m with steel poles. This type of barrier is located only on bridge structures. A regulated artificial noise source was chosen as the noise source (Figure 5), which was used for both measurements, i.e., for measurements without a barrier and with a barrier. It was an omnidirectional simple point source with pink noise, which was situated in the middle of the road during the measurement. The acoustic average was used to calculate the insertion loss of the noise barrier. Figure 4: Diagram of the set-up for reference measurement in the free sound field to determine the sound insulation index [29]; the symbols used in Figure 4 are defined in Figure 2.  
Figure 5: Regulated artificial noise source. The location of the measuring points according to [28] is schematically shown in Figure 6a,b) and in situ in Figure 7. The duration of measurements was 2 min in accordance with [28] due to the use of an artificial steady noise source. \(L_{\textit{ref},B}\)--sound pressure level at the reference point without the barrier, microphone located at a height of 1.5 m above the level of the barrier, at an equivalent point with the place of installation of the barrier. \(L_{\textit{r},B}\)--sound pressure level at the reception point without the barrier, microphone located from the reference point \(L_{\textit{ref},B}\) at a distance of 1.0 m behind the place of installation of the barrier and at a height of 1.5 m above the road surface. \(L_{\textit{ref},A}\)--sound pressure level at the reference point with the barrier, microphone located at a height of 1.5 m above the barrier. \(L_{\textit{r},A}\)--sound pressure level at the reception point with the barrier, microphone located at a distance of 1.0 m behind the place of installation of the barrier and at a height of 1.5 m above the road surface. In Table 3, the climatic conditions during the measurement are given. Figure 6: Diagram of the set-up of the measuring points and noise source for the indirect method: (**a**) measurement without a barrier and (**b**) measurement with an installed barrier (distance from the source 5 m). Figure 7: Measurement of the noise barrier insertion loss SO 241-00: (**a**) location of the microphone above the barrier \(L_{\textit{ref},A}\) and (**b**) location of the microphone at the reception point \(L_{\textit{r},A}\). The following instrumentation was used for the measurement: Integrating--averaging sound level meter SVAN 979 2 pcs, Measuring microphone G.R.A.S--40 AE 2 pcs, Calibrator Nor 1251, Thermal anemometer T405-V1. We used pink, white, and pink/white noise as the excitation signal using a Nor-280 amplifier and a Nor-276 all-directional dodecahedron speaker, which emits an acoustic power level \(L_{W}=120\) dB. The results of the measurement and calculation according to Equations (5)-(7) are given in Table 4 and a diagram of the sound pressure levels in the range of the middle frequencies of the one-third octave bands and single-number rating of sound insulation with a barrier and without a barrier are presented in Figure 8 (reference values) and in Figure 9 (levels at the reception point). It is important to realize that the reference point is situated above the barrier (Figure 6).