## 1 Introduction
The static or quasi-static stiffness measure is well-consolidated technique [28] and is related to the stimulation of the sample at very low frequencies (less than 25 Hz [29]). In contrast, the dynamic stiffness (DS) measure is related to the frequency response of the pavement at higher frequencies (typically around the range of interest of rolling noise), and its technique is yet to be perfected, both in situ and in the laboratory. In the last decade, several articles were published trying to adapt preexisting measurement techniques to pavements [30] or to develop novel techniques or novel indicators [18]. One of the most widely adopted methods is that indicated in ISO 9052-1:1989 [31] for measuring the stiffness of resilient materials, which are characterized by lower levels of DS compared to pavements. The measurement technique proposed by the standard is called resonant, as it is based on the mass-spring system. The test sample acts as a spring while the role of the mass is performed by a metallic plate; thus, by knowing the resonance frequency of the system, it is possible to derive the stiffness of the sample. Subsequently, studies have been conducted using a measurement technique called non-resonant [32], which can be seen as a sub-case of resonant measurements where the ratio between the sample's stiffness and the weight of the load plate is big. In this regime, by stimulating the sample with a force and measuring its response in terms of displacement, it is possible to derive the stiffness of the material. The stiffness's frequency response is obtained by stimulating the specimen at multiple frequencies. Using samples of elastomeric material, it has been experimentally demonstrated that this method is equivalent to the resonant one [33]. Furthermore, due to the fact that it does not require the installation of a loading mass, the non-resonant method is more practical. Both of those techniques can be executed by two different instrumental chains for two different force signal generation. The simplest chain implies the usage of an impact hammer that allows for generating impulsive force signals (ISO 7626-5:2019 [34]). The most complex one, already established as the standard measurement chain in the field of modal analysis, implies the usage of a dynamic exciter or mechanical shaker, also known as \"shaker\", that can generate sinusoidal and pseudo-chaotic force signals ISO 7626-2:2015 [35]. Earlier work applying these techniques to road pavements used an impulsive force. Following results that are not too encouraging, some authors introduced the use of the shaker, which by generating sinusoidal signals allows for a higher signal-to-noise ratio. Then, after a number of studies utilizing the shaker were carried out, some authors [36] have successfully applied the non-resonant technique to road pavements and have proposed a model for the frequency response of DS. They, however, argue that the same results cannot be obtained using the instrumented hammer. The shaker's ability to perform the sinusoidal excitation of the sample allows is to reach high level of signal-to-noise ratio, very good stability, and good repeatability of the measurement. However, it presents some critical issues that make its usage not very practical for on-field measurements. First of all, it is expensive, difficult to transport, heavy, and requires a powerful and heavy amplifier. Secondly, to cover the desired spectrum of the DS, multiple measurements are needed or a sweep signal must be utilized, requiring a long time to execute the measurement. The alignment of the shaker is also time consuming. On the other hand, the impact hammer generates impulses that, in theory, are capable of investigating a wide range of frequencies in one shot, and so, with far less time compared to the shaker. The hammer is also easier to transport, less expensive, and easier to use. Moreover, shaker and hammer measurements were proven to be equivalent for the measurement of DS for other types of materials [37; 38], so the same should be possible for pavements.