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In order to optimize the performances of the thermoacoustic refrigerator working with the high sound pressure level, the nonlinear acoustic characteristics of the thermoacoustic stack in the resonant pipe are studied. The acoustic fluid impedance of the stack made of copper mesh and set up in a resonant pipe is measured in the acoustic fields with different intensities. It is found that when the sound pressure level in the pipe increases to a critical value, the resistance of the stack increases nonlinearly with the sound pressure, while the reactance of the stack keeps constant. Based on the experimental results, a theory model is set up to describe the acoustic characteristics of the stack, according to the rigid frame theory and Forchheimmer equation. Furthermore, the influences of the sound pressure level, operating frequency, volume porosity, and length of the stack on the nonlinear impedance of the stack are evaluated. [ABSTRACT FROM AUTHOR]

The tubular acoustic metamaterial with the double negativity based on the membranes and side holes periodically distributed along a pipe is studied with the Bloch theory. The results are verified by the full-wave simulation method and are used to explain the related experimental phenomena: the low-frequency forbidden band starting from the zero Hertz and the relations between the cutoff frequencies of the forbidden and pass bands in the single negativity metamaterials and the double negativity metamaterial composed of them. Based on the Bloch theory, the structural parameters of the membranes and side holes in the metamaterial are optimized to achieve the maximum pass band with the negative phase velocity (PBNV) since the PBNV is determined by the coactions of the membranes and side holes. Furthermore, the structures of the metamaterials are improved in order to broaden the PBNV further and achieve the balanced condition simultaneously. [ABSTRACT FROM AUTHOR]