# A Review on Vibration Control Using Piezoelectric Shunt Circuits
## 3 Experimental Studies
The correctness and the feasibility of the semi-active vibration control method are verified by experiments. The superiority of the proposed shunt circuit is demonstrated experimentally with the comparison groups including a depolarized comparison group and the common shunt circuits. [PERSON] et al. [110] proposed an adaptive piezoelectric metamaterial beam (piezo-meta-beam) that consists of bimorph piezoelectric arrays. The shunt circuits are designed with self-tuning abilities by integrating microcontroller-driven digital potentiometers into synthetic inductive circuits. Two typical scenarios are considered: harmonic and white noise excitations with different spectra. A series of experiments is conducted to investigate the adaptive behavior of the piezo-meta-beam. In the harmonic sweep excitation test, the adaptive piezo-meta-beam shows an ultra-broad attenuation zone (220-720 Hz), while the traditional counterpart only has a bandgap width of less than 20 Hz. In the case of noise excitation, autonomous adjustment of the center frequency and attenuation zone is achieved for noises over different spectra. In general, this work presented a methodology for designing intelligent metamaterials that can adapt to environmental vibrations with vast potential for real applications. [PERSON] et al. [111] proposed a new nonlinear piezoelectric shunt that is based on the intentional use of a bilinear non-smooth component in the electrical circuit. The introduced piezoelectric shunt circuit enabled exploiting the non-smooth component to enhance the resonant shunt through the significant reduction in the required inductance without losing the performance. The inherent nonlinearities can activate the 2:1 internal resonance, which can be useful for applications involving energy harvesting. Indeed, due to multi-harmonic response, the non-smooth term allows the activation of other types of internal resonance. The non-smooth term was easily analyzed with a diode in practice, leading to a shunt circuit that can be easily controlled and tuned. The proposed absorber can pave the way for future studies involving the possibility of adding quadratic and cubic nonlinearities in combination with the non-smooth component to activate the 2:1 internal resonance. Such a design can possibly lead to a response similar to an RL-shunt, but associated with a saturation phenomenon. [PERSON] et al. [112] presented a theoretical model of panel-cavity-panel system (PCPS) controlled by piezoelectric shunt oscillators (PSOs) and its vibration equations. The vibroacoustic characteristics of PCPS, including sound absorption coefficient (SAC), energy loss coefficient (ELC), and transmission loss (TL), are calculated using a unified mathematical form based on the vibroacoustic radiation impedance matrix, damping matrix, and displacement response of PCPS, while a lumped parameter model of PCPS is derived to simplify the comprehension and extension of PCPS. Experiments of PCPS controlled by PSOs with negative capacitors are conducted in a standing wave tube, at non-eigenfrequencies ranging from 70 Hz to 220 Hz. It was found in experiments that PSOs with negative capacitors can increase the TL of PCPS about 8 dB to 25 dB, which depends on the target frequency. SAC and ELC can also be increased, which means less acoustic energy is reflected and more acoustic energy is dissipated in the shunt circuits. [PERSON] et al. [113] investigated a smart side-branch silencer controlled by shunted piezoelectric resonators. A three-dimensional finite element model is constructed to study the effects of damping and electromechanical coupling on the proposed smart silencer, and experiments are conducted to test the performance of this silencer. Both simulated and measured results demonstrate that extra resonant peaks can be added from the electrical resonances without significant negative impacts on the sound attenuation associated with the plate resonances.