# Shunt Damping Vibration Control Technology: A Review
## 7 Challenges in Shunt Damping Vibration Control  
### Noise and Acoustic Control  
Noise and acoustic control from vibrating structures is a key point in various fields such as automobiles, airplanes, ships, submarines, and buildings. There have been many attempts to reduce or control the noise radiating from structures. [PERSON] and [PERSON] [128] applied the RLC resonant shunt to increase the acoustic transmission loss of a plate shown in Figure 16. A variable inductor was used to tune the frequency of the circuit and it was implemented based on the operational amplifier circuit. The experimental results showed that 7 dB increase of transmission loss can be achieved for a broad band input of 10-10,000 Hz. [PERSON] and [PERSON] [129] also used a resonant shunted piezoelectric transducer on panel structures to reduce the radiated noise. [PERSON] et al. [130] used the passive in-series resistor-capacitor shunt of an electrodynamic loudspeaker efficiently to realize a single resonant mode attenuation in a narrow frequency range. However, the resonant circuit is limited to the frequency and the absorbing bandwidth is narrow. Consequently, [PERSON] et al. [131] employed a negative capacitance shunt circuit for broadband noise control of a piezoelectric smart panel. The acceleration levels at the fifth, ninth, 11 th, and 21 st modes were reduced by 15 dB, 7 dB, 4 dB, and 3 dB, respectively. The transmission loss was increased by 7-2 dB at 500-2000 Hz. Figure 16: RLC shunted piezoelectric transducers for increasing the transmission loss of a plate, reproduced with permission from [128], Academic Press, 2001. Figure 15: Adjustable negative capacitance shunt circuit. [PERSON] et al. [111] presented the general approach of noise and vibration control using piezoelectric materials and negative capacitance circuits. The results implied that the acoustic absorption coefficient of the sound absorption system can be controlled. The transmission loss can be reduced by 7 dB in the broad frequency range. This method is called active elasticity control (AEC). [PERSON] et al. [112] studied sound isolation also using a piezoelectric polymer film connected to a negative capacitance. The transmission loss can be reduced by 13 dB at frequencies around 1 kHz. [PERSON] et al. [132] studied the sound-induced vibration control using a piezoelectric composite sheet connected to a negative capacitance shunt. In order to overcome the discrepancy of the external and internal capacitance, [PERSON] et al. [114] proposed an adaptive negative capacitance shunted piezoelectric membrane to improve sound shielding efficiency. The shunt circuit can be found in Figure 14.  
[PERSON] et al. [133] presented the theory unifying passive and active acoustic impedance control strategies, introducing the concept of electroacoustic absorbers. The equivalence between shunts and active control was formalized through the introduction of a one-degree-of-freedom acoustic resonator accounting for both electric shunts and acoustic feedbacks. Broadband acoustic performances have been obtained by using the resistance, negative resistance shunt and direct impedance control. [PERSON] and [PERSON] [134] considered that the real and imaginary parts of the shunt circuit impedance must be negative in the considered system to obtain a perfect sound absorption in a broad frequency range. The results implied that it is possible to decrease the acoustic reflection coefficient at an arbitrary frequency by proper adjustment of the parameters of the negative resistance shunt. [PERSON] et al. [135] demonstrated effective broadband sound absorption by reducing the mechanical reactance of a loudspeaker using a negative inductance negative resistance shunt circuit through electro-mechanical coupling, which induces reactance with different signs from that of loudspeaker. The results implied that the sound absorption coefficient is well above 0.5 across frequencies between 150 and 1200 Hz. [PERSON] et al.