## 1 Introduction
The relative simplicity of the passive vibration control system makes it more reliable and robust [19]. The passive vibration control method is achieved by a mechanical connection that limits the vibration amplitude of the connected or coupled structures [20]. The mechanical connection dissipates or redirects the vibration energy before the energy is transmitted to the structures to be isolated. Figure 2 shows a prototype of the passive vibration control system [21]. The most common passive vibration control method is to move or tune the system-resonant frequencies away from the excitation frequencies. The system resonant frequency change mainly results from changing system stiffness and mass. The reduction in system resonance peak amplitude results from adding layers of damping materials. Figure 1: Active seating suspension system prototype [16]. In some cases, electromechanical controls can sometimes be applied to optimize the system isolation, while the isolation mechanism itself is passive [22]. There are different isolation components adopted in passive vibration control systems such as fluids, springs, elastomers, and negative-stiffness structure components [23]. The spring is one of the most essential components, and it is placed between a source transmitting vibration and the receiver to be isolated. The spring isolates the impulse force above it and absorbs some vibration energy as it deforms [24]. In order to obtain better isolation performance, in addition to the spring isolation element, an elastomer is also used to achieve the damping effect. Elastomers are rubber-like materials that can absorb mechanical vibration energy through deformation. While the rubber has different deformation characteristics in different directions, the isolation performance would be different in different directions. Other isolators such as air, negative-stiffness, and magnetic structures are also sophisticated passive vibration control isolators [19]. Three types of the vibration control systems mentioned above aim to improve the ride comfort of the seating suspension system and ensure the health of the vehicle operator and passengers. The vehicle driver usually feels uncomfortable vibration in the low-frequency range from 2 to 8 Hz, as the vibration is more likely to affect work efficiency [25]. [PERSON] et al. studied the comfort and ergonomics of seating suspension systems from several different perspectives in 1992 [26]. Several methods including electromyography, intervertebral disc pressure measurement [27], vibration transmissibility [28], pressure distribution at the occupant's seat interface [29; 30], and microclimate at the occupant's seat interface [31] have been adopted to quantify ride comfort. As the research on the ride comfort of seating suspension systems goes further, new concepts and methods will be found. Low-frequency excitation generates whole-body-vibration (WBV), which affects all parts of the human body [32]. Multiple vibration transmission directions through a vehicle seat occupant are given in Figure 3[33]. It clearly shows that the three translational vibrations are along the \(x\), \(y\), and \(z\) axes and the three rotational vibrations are marked as roll, pitch, and yaw around the \(x\), \(y\) and \(z\) axes, respectively. Several international industrial standards and safety manuals have been given to define a criterion for assessing the vibration dosage [33], which includes The Australian Standards AS-2670 [34], International Standards ISO-2631 [35], and British Standards BS ISO-2631 [36]. Several factors are adopted to evaluate the dosage of WBV. Different vibration isolation systems have been designed and developed by different researchers in the past and will be introduced and illustrated in the following sections. Figure 2: Passive seating suspension system prototype [19].