## 3 Literature Review  
In Table 1, a synthetic summary of the techniques and the instruments utilized by various authors from the literature is reported. As it can be seen, there is not a pronounced preference to one technique or instrument, but the most recent articles seem to prefer the utilization of a mechanical shaker instead of hammer. Furthermore, the non-resonant technique is the most used. The first article to the authors' knowledge that reports measurements of the DS of road pavements is [39]. In this work, the dynamic stiffness of four specimens of SMA pavements was tested by adapting the ISO 9052-1:1989 [31] to perform comparisons between results from static measurements based on EN 12697-26:2018 [28] and between results from the CPX measurements performed on the corresponding in situ paving of the same pavements. The tested pavements are differentiated by their maximum grain sizes (11 mm or 16 mm) and by the presence of crumb rubber. The frequency range of analysis for DS measurement is below 500 Hz. For three of four specimens, the results appear to indicate a correlation between the static and the dynamic results, but for the forth one (11 mm with no crumb rubber), the DS is below expectation. The CPX results were tested on a test track to evaluate noise emission from SMA 16 mm and SMA 11 mm, but the authors do not indicate if crumb rubber is present or not in the mixture. The results seem to confirm a lower emission from the 11 mm samples that resulted in lower DS, but the anomalous result of the 11 mm with no crumb rubber raises doubts about the results. In a following work [32], the authors performs DS measurements in resonant and non-resonant configurations by mean of a mechanical shaker. The specimens tested are described in Table 2, where the corresponding values of the DS, derived by the authors by multiplying the resonant results presented in [32] using the sample area, are also presented. The value was calculated to allow for a direct comparison between the results of the two configurations. In comparing the values, we must keep in mind that the author's analysis of the spectrum was limited to 500 Hz because at this frequency, there was a resonance that seemed to invalidate the measurement for higher frequencies. As we can observe, there \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Article** & **Resonant** & **Non-Res.** & **Hammer** & **Shaker** & **D. Stiffness** & **M.** \\  & & & & & & **Impedance** \\ \hline [PERSON] 2012 [39] & Yes & No & Yes & No & No & No \\ [PERSON] 2014 [40] & No & Yes & Yes & No & Yes & Yes \\ [PERSON] 2014 [41] & No & No & No & No & Yes & No \\ [PERSON] 2016 [32] & Yes & Yes & No & Yes & Yes & No \\ [PERSON] et al., 2016 [18] & No & Yes & Yes & No & No & Yes \\ [PERSON] 2019 [42] & No & Yes & No & Yes & Yes & No \\ [PERSON] 2020 [36] & No & Yes & No & Yes & Yes & No \\ [PERSON] 2020 [43] & No & Yes & No & Yes & Yes & No \\ [PERSON] 2023 [44] & No & Yes & Yes & No & No & Yes \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Literature articles on pavement stiffness or MI evaluation with an indication of the techniques and instruments used. is no agreement between the results for the softer materials (PU and EPS) as the values obtained with the two techniques are of different orders of magnitude. This result would be in disagreement with what was obtained by [33], which found substantial equivalence between the two methods. A better agreement seems to be obtained for the remaining samples; however, the authors do not provide estimates of the uncertainty, so it is not possible to make a judgment on the merits of this method. A subsequent paper [36] has refined the application of the non-resonant method to road pavements and has applied it to in situ pavements. The data provided by the authors on the tested pavements are shown in Table 3. The authors extend the analysis up to 4 kHz, obtaining the same resonance evidenced in their previous works [32; 42] and by other authors [18].