Extraordinarily rapid mixing has surprisingly been achieved in confined mixing layers by active forcing within an optimal narrow frequency band (Wang, PhD Dissertation, 1999; Wang, Chem. Eng. Sci., 2003; Wang, AlChE J., 2006). However, the corresponding mechanism of the large receptivity and rapid mixing is still unclear so far. Since it is found that the optimal frequency that corresponds to the rapid mixing is independent of Reynolds number or bulk flow velocity, we postulate that some type of resonance of the experimental setup or fluid-structure interaction might be the cause. In this manuscript, the possible influence of acoustic resonance and fluid-structure interactions are investigated by both parametric study through flow visualization and finite element method (FEM). The results reveal the optimal forcing frequency of the fast mixing is tightly related to a complicated acoustic resonance mechanism of the entire water tunnel system, and when the confined mixing layers are forced under the resonance frequency, ultrafast mixing can be observed. This finding may lead to new technology for mixing and heat transfer enhancement. (C) 2016 Elsevier B.V. All rights reserved.