## 2 Vibro-Acoustics Characterization  
The determination of the true vibro-acoustic characteristics of a structure is essential for the correct prediction of the actual vibration and sound radiation behaviors of the structure. Therefore, because of its importance, the vibro-acoustic characterization of structures has drawn close attention. [PERSON] et al. [1] determined the vibro-acoustic characteristics of a sandwich plate consisting of phenolic open-cell foam core using both theoretical and experimental methods. The vibro-acoustic characteristics were then used in [PERSON]'s sandwich beam bending model to calculate the sound transmission loss of the plate. For verification, they made a comparison between sound transmission loss predictions and measurements in sound transmission suites according to ISO 10140-2. The suitability of [PERSON]'s mathematical model was also investigated via the use of the nominal dimensions of the constituent layers to calculate the sound transmission loss of the sandwich beams. The calculated results have shown that the dimension variations of the constituent layers induced in the manufacturing process may affect the accuracy of the model. [PERSON] et al. [2] studied the effects of vibro-acoustic characteristics on the average sound radiation efficiency of a sandwich plate with periodically local resonant (LR) units. In the vibro-acoustic characterization, they first derived closed form solutions to study the band-gap properties of an infinite periodic structure and the vibrational response of a finite periodic structure. They then used the concept of modal radiation to calculate the acoustic radiation efficiency of the LR sandwich plate. The theoretical results show that the acoustic radiation power can be reduced significantly not only in the band-gap but also at the frequencies in the neighborhood just below the band-gap. For frequencies in the neighborhood just above the band-gap, the acoustic radiation efficiency did not decrease but rather increased. They also studied the effects of damping and effective masses at the LR units on the sound radiation efficiency of the sandwich plate. It has been shown that the addition of small damping to the resonator can reduce the sound radiation at the frequencies above the band-gap and thus further broaden the attenuation frequency band. It has also been shown that an increase in effective mass corresponds to a decrease in radiation efficiency and vice versa. Therefore, it has been demonstrated that the periodic resonators may have the capabilityof providing a broader attenuation band for the purpose of noise reduction. [PERSON] et al. [3] studied the vibration characteristics of a dual-bimorph piezoelectric pumping element under fluid-structure coupling via both theoretical and experimental approaches. They used amplitude-fluctuation electronic speckle pattern interferometry and a laser Doppler vibrometer to measure, respectively, the visible displacement fringes and point-wise displacements. It has been shown that the experimental resonant frequencies and the corresponding mode shapes are in good agreement with the theoretical results obtained using the finite element method. The experimental results were also used to elucidate the anti-phase as well as the in-phase motions associated with vibration. It has been shown that the gain in flow rate obtained via the anti-phase motion of the dual-bimorph pumping element is larger than those obtained via the in-phase motion and using the single bimorph pumping element. It has been pointed out that the vibration characteristics of the piezoelectric pumping elements with two bimorphs may provide valuable information for the future development of bionic pumps. [PERSON] et al. [4] studied the vibro-acoustic characteristics of circular plates embedded with two-dimensional acoustic black holes (ABHs) via both theoretical and experimental approaches.