Damping in viscoelastic materials such as rubbers is often desirable, especially in loudspeaker suspensions. Under high strain loads however, viscoelastic materials can also exhibit a hysteretic stiffness behavior, causing a stiffness decrease with amplitude. In this study, we examine the viscoelastic rubber suspension of a loudspeaker, using the loudspeaker motor system as actuator and sensor. From measurements we observe the hysteretic force-displacement behavior and pronounced odd-order harmonic distortion even at low amplitudes, in accordance with the literature. We further explore a macro-thermodynamic plastic flow model to model the stiffness of viscoelastic materials. The results show that the plastic flow suspension model explains and replicates the observed nonlinear hysteretic behavior. We also show that a fitted time-domain loudspeaker model including plastic flow matches the measured distortion profile. In contrast, models with polynomial stiffness and viscous damping fail to explain the observed amplitude dependencies such as odd order harmonic levels. The experiments demonstrate that viscoelastic hysteresis occurs not only at high but also at low amplitudes, where the elastic stiffness is approximately linear.
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