## 3 Materials and Methods  
The material used for the tests features a 24-mm hexagonal aluminum honeycomb core sandwiched between two identical 1-mm aluminum laminates. Figure 6 shows a picture of the beams, while Table 2 summarizes the main geometrical and weight properties of the beams tested according to the method described in Section 2.4.  
To derive the natural frequencies of beam test samples, the beams have been suspended through elastic strings (Figure 5). An accelerometer PCB Type 352A24 has been placed at one side of the beam, while the specimen has been hit using an impedance hammer PCB Type 086E80 with a plastic tip at the other end. The transducers have been connected to an OROS OR 36 analyzer. The software managing the analyzer can compute the frequency response function in real time. Once the eigenfrequencies are derived, the bending stiffness at a particular natural frequency can be computed by using Equation (21).  
From the same panel the two beams have been cut from, a specimen had previously been obtained and tested in sound transmission suites. Figure 7 shows a picture of the panel ready to be tested according to the standard ISO 10140-2 [36]. The sound transmission loss has been determined on a 1.70 m \(\times\) 1.09 m specimen. In order to avoid sound leakage through the sides of the specimens fit into the wall dividing the two hard rooms (Figure 7b), silicone has been used to seal the air gap between the wood frame of the opening and the plate. A second wooden frame has been used to close the perimeter of the panel and avoid lateral transmission through the edge. An omnidirectional sound source has been placed into the emitting room and a pink noise generated. The sound pressure levels in the emitting and receiving rooms have been measured by using an L&D 824 sound analyzer and subsequently space-averaged. The reverberation time has been measured using the interruption  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Characteristics** & **Symbol** & **Units** & **x-Direction** & **y-Direction** \\ \hline Length & \(L\) & m & 1.803 & 0.993 \\ Core thickness & \(h_{c}\) & m & 0.024 & \\ Laminate thickness & \(h_{I}\) & m & 0.001 & \\ Width & \(b\) & m & 0.050 & \\ Mass per unit area & \(\mu\) & kg m\({}^{-2}\) & 6.72 & \\ Young’s modulus of the laminates & \(E_{I}\) & GPa & 72 & \\ Cell size & - & in & \(3/8^{\prime\prime}\) & \\ Core density & \(\rho_{c}\) & kg m\({}^{-3}\) & 46 & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Characteristics of the beams used for natural frequency measurements.  
Figure 6: Beams used for natural frequency measurements (\(x\)- and \(y\)-direction).  
of a stationary pink noise. During the session, several structural reverberation time measurements have been performed in order to determine the loss factor. The signal has been recorded by using a PCB 352A24 accelerometer connected to an OROS OR 36 system. The panel has been hit close to the accelerometer position with a PCB impedance hammer Type 086C80 equipped with a nylon tip. The recorded signal has been further post-processed using Adobe\({}^{\cir  
the sample featured the whole sandwich structure, and the clamps have been bonded directly to the external aluminum laminates by a professional adhesive. Pictures of the tested sample and of the experimental setup are shown in Figure 8.