## 1 Introduction
To have a very low dynamic stiffness at the notch frequency, thus, lower cabin noise and vibration, rubber damping, and inertia track flow losses must be low, and the fluid inertia (mass of the fluid in the inertia track) of the track must be high. The limited amplitude sensitivity of the hydraulic engine mount design shown in Figure 2 is solely attributable to rubber's strain sensitivity. To make the hydraulic or fluid-filled engine mount significantly more amplitude sensitive, decouplers [8; 9; 10; 11; 12] are employed, as shown in Figure 4.  
As seen in Figure 4, hydraulic engine mounts with decouplers typically use two inertia tracks: the spiral-shaped inertia track and the center inertia track with a decoupler. The majority of hydraulic engine mount solutions with decouplers operate more like a fluid damper rather than a vibration isolator. The center inertia track has a large diameter due to the decoupler and its length is short; thus, the fluid inertia, mentioned on page 2, is very low. When the engine amplitudes are low, the fluid flows around the decoupler, moves the decoupler up and down, and only a little amount of fluid passes through the spiral-shaped inertia track; thus, the flow losses are significant, and the fluid inertia is very low. Thus, the hydraulic engine mount will exhibit a very shallow notch or no notch frequency at all, act like a fluid damper rather than a vibration isolator, and its dynamic stiffness will be, in general, low. When the engine motions are high due to shock, the decoupler, which is floating, blocks the center fluid inertia track, and fluid flow will be through the spiral-shaped inertia track. When the spiral-shaped inertia track is active, there is fluid inertia, but since the engine displacement amplitudes are high, the spiral-shaped inertia track flow losses will  
Figure 4: A hydraulic engine mount with a decoupler. Figure 3: Dynamic stiffness of a typical hydraulic engine mount versus frequency. be also high. Therefore, a shallow notch or a hardly present notch will be observed in the dynamic stiffness curve. At high engine displacements due to shock, the engine mount feels stiff; thus, the engine mounts are able to control engine motions. The decoupler will again float once the shock has subsided and the engine motions have returned to being minimal. The existence of the decoupler, in most cases, eliminates the notch frequency; thus, often, the hydraulic or fluid-filled engine mounts with decouplers no longer act as a vibration isolators but as a hydraulic dampers. The experimental dynamic stiffness plot shown in reference [1] clearly shows that at low engine motions, \(\pm 0.1\) mm, the dynamic stiffness is flat with no notch at all, and at \(\pm 1\) mm engine motions, a shallow notch is observed. A similar phenomenon is seen in the dynamic stiffness plot of Figure 5a of reference [13]. In the dynamic stiffness of Figure 3 in reference [12], at small motions, a shallow notch is seen, and at large engine motions, no notch is seen. In reference [14], a rigid decoupler plate is replaced with a flexible membrane and the tension of the membrane is adjusted, thus adjusting the hydraulic engine mount dynamic stiffness. In the proposed design of reference [14], the decoupler no longer floats. The paper mentions that the flexible decoupler can be made from a thin metal membrane, and its stiffness/tension can be adjusted by using PZT patches. The work of reference [14] is very interesting but requires PZT patches, a power supply, sensors, a controller, and a control policy to detect when to increase or decrease decoupler tension. The work is mostly theoretical, and the proposed design has not been verified experimentally. In this paper, it is shown that the dynamic stiffness can be adjusted as membrane tension is adjusted, but this design is unable to adjust dynamic stiffness passively and automatically as engine displacement amplitudes are increased.