Nonlinear impedances of thermoacoustic stacks with ordered and disordered structures

Nonlinear impedances of two thermoacoustic stacks with ordered structures (plate-type and pipe-type) and one with a disordered structure (copper mesh) are studied. The linear resistances, nonlinear coefficients and effective acoustic masses of the stacks are extracted from the experimental results based on an analogical model of nonlinear impedances of porous materials. The resistance and nonlinear coefficient of the disordered stack are found to be much larger than those of the ordered stacks, which have similar volume porosities. In the ordered stacks, the resistance is only marginally influenced by the length of the stack, while in the disordered stack, the resistance increases significantly with the length. These characteristics of the impedances of ordered and disordered stacks are explained with the minor loss theory and the tortuosity of a stack.