## 9 Conclusions  
The DS is an essential parameter for the characterization of pavements, not only in terms of durability but also in terms of noisiness. Indeed, different studies suggest a correlation between DS and the noisiness of pavements, suggesting a lower DS is connected to lower levels of TRN. However, the DS measurement is not straightforward because of the typically high level of pavement stiffness. Thus, the measurement technique requires a fine-tuning process. In the literature, the _slaker_ is considered to be the reference for those measurements, while the impact hammer is considered incapable of producing acceptable results due to the lower signal-to-noise ratio that is technically achievable. Indeed, using the shaker, some authors obtained good results with a non-resonant technique in measuring the DS of in situ and in-laboratory pavements, proposing a model for the DS spectrum [36]. The present work had the aim of fine-tuning the impact hammer measurement technique for DS to obtain DS spectra analogous to those obtained in the literature with the _shaker_. For this purpose, a low-cost 3D-printed device named TAD was developed to allow for the better control of the force impressed and impact angles, allowing the identification of the parameters have the greatest impact on the quality of the results. The system, successfully calibrated in the 100-1000 Hz range, was then able to evidence that the most critical aspect of the impact hammer measurements of pavement DS can be found in the manual usage of the hammer, highlighting the strong influence of the impact angles. Consequently, using the TAD, out work focused on optimizing the response detection, also studying the influence of the fastening process (for concrete and plaster), the hammer tip, the applied force, and the presence of asperity on the impact area. This process led to the definition of a final configuration of the system for in situ and in-laboratory measurements. In parallel with the hardware optimization process, the analysis of the results was also improved to bypass the criticality that emerged at high frequencies. In this way, it was possible to successfully perform the fit with the DS model proposed by [36], which as a result, consistent with the literature, was not possible to be obtained by the manual use of the hammer. The developed system was then utilized to perform in situ and in-laboratory DS measurements on rubber-modified bitumen pavement. The results of those measurements confirmed the capability of the system to reproduce the predicted DS spectrum. In the meantime, some criticalities of the in-laboratory measurements emerged in comparison with the in situ measurements. Indeed, the latter seems to exhibit a wider frequency range and lower oscillations in the spectrum, suggesting the greater reliability of the results. On the other hand, working on finite dimension specimens, in-laboratory measurements have the advantage of also being able to estimate the Young modulus of the sample. The optimization work carried out paves the way for the usage of the impact hammer instead of the shaker, with undoubtedly advantages in terms of investment costs and the required time for perform the measurements. In the future, by using the hammer, it would be possible to carry out larger measurement campaigns to better investigate the dynamic stiffness of pavements. Either way, while representing an important step forward for pavements' DS measurements with an impact hammer, the present work has some aspects that may benefit from further investigations. For example, the achieved calibration, while consisting of a good starting point and which being a good candidate for in situ calibrations, is far from perfect. Indeed, the possibility of extending the calibration interval can be investigated in future studies. Another aspect that will require future investigations is the direct comparison between the shaker and the TAD results in terms of accuracy and precision.