Low frequency sound absorption and transmission loss of a tortuous waveguide-resonator system: Theory vs Experiment

This work investigates, theoretically and experimentally, the low-frequency acoustic behaviour of tortuous waveguides laterally loaded by acoustic resonators in reflection and transmission conditions. A homogenisation-based model has been used to design prototype metamaterials prior to experimental validation. The metamaterials exhibit a tunable sound absorption peak at a frequency significantly lower than the resonance frequency of the loading resonators. It is evidenced that an increase in the tortuosity of the waveguides can allow a reduction in thickness of the metamaterial, while maintaining or improving the key features of the low frequency absorptive spectrum. It is also shown that the addition of thin porous layers to the metamaterial can improve both the amplitude and bandwidth of the sound absorption peak while also controlling sound transmission via apparent visco-elastic effects.