## 8 Discussion  
From the tests performed to find the critical aspect of the manual non-resonant technique for the DS measurement of pavements emerged that the use of the TAD provides much better results. The interesting parameters of the impact are basically three, the maximum force impressed, and the two angles of incidence with respect to the normal axes of the plane. However, in fields such as the modal analysis and measurement of soft materials according to ISO 9052-1:1989 [31], these parameters do not affect the measure in a way that would invalidate the result. Investigating the reasons for this difference, it was found that the most critical parameters are the impact angles. Indeed, the linearity test demonstrated that force does not influence the measured SD; therefore, the difficulties in impressing a consistent force by hand is not an issue. The results showed that the non-zero angles of impact occurring in the manual usage of the impact hammer could affect the DS spectrum, resulting in increased uncertainties that are arise while fitting the model. Indeed, with a bigger angle of incidence, the impedance head does not operate properly, as some of the energy is delivered along the orthogonal directions to the head axis. The TAD, by allowing the hammer to be leveled, ensures that it impacts the head surface at a small angle, which remains constant throughout the measurement session. In this way, the TAD maximizes the \(R^{2}\) and minimizes the confidence interval of the estimated value for the DS of the sample. More importantly, TAD allows us to obtain the DS spectra and the associated uncertainties in accordance with the model proposed in the literature [36]. Regarding the sample fastening method, we tested the possibility of using concrete instead of plaster. In terms of linearity, concrete seems to perform better at 100 \(N\) and above, while plaster performs better for lower impressed forces. Looking at the drying process monitoring results presented in Figure 13, the plaster seems to yield better results in the investigated drying period (from day one to day three). When judging these results, one must considered that the drying performance of plaster and concrete varies considerably based on external temperature and humidity. Indeed, the plaster is advantaged with higher temperatures and lower humidity while the concrete dries faster in high humidity. Thus, the right choice of fastening material depends on the external conditions. Concerning the hammer tip, Figure 14a shows that negligible differences are present between rubber and nylon tips regardless of the difference indicated by the manufacturer and reported in Table 6. However, the rubber tip was identified as the preferred one due to the lower level of uncertainties associated with the third-octave band's values. In view of the results from the study on the samples fastening process, it must be underlined that the in-lab measurements are subjected to problems due to the curing time of the cement/plaster and to accidental detachment phenomena that can compromise the measurement. In the case of in situ measurements, the stabilization of the sample is not an issue, so the main criticalities are connected to any slopes in the pavement. When the road has a remarkable donkey saddle profile, non-zero impact angles can occur. In those cases, the correct alignment of the instrumentation is challenging, and more attention must be paid. Regarding the impedance head, the presence of the threaded hole on top of the top face generate important variation in the spectrum and higher uncertainties, as shown in Figure 15a. The installation of a metallic interface on top of the head is thus crucial in producing correct DS measurements. Later, further DS measurements were performed on two other different test pavements, a pavement sample, and a in situ pavement. Those results showed good agreement with the DS model proposed by [36]. Moreover, the absolute DS value and the associated standard deviation are comparable with those obtained by [36] using the shaker.