Road noise varies with the vehicles’ load and speed, ranging from low to medium frequencies. The existing absorbers, including acoustic metamaterials, cannot fully mitigate the variable noise due to limited bandwidth. To target the variable noise but not occupy much space, this paper presents a tunable acoustic absorber that can transform from Helmholtz resonance to membrane resonance. This tunable absorber consist of double micro-slit layers, namely an electro-adhesive membrane and a flexible plate spaced closely in front of a shallow cavity. The boundary-layer thick gap between the two forms a network of lateral orifices to enhance the acoustic damping at medium frequencies. When the gap closes under electrostatic attraction force, the membrane and plate zipped together and resonated like a drum at a much lower frequency. This transformation can shift the resonant frequency down by 1.6 octaves from 898 hetz (Hz) to 281 Hz, thanks to the structural resonance of the zipped parts. Interestingly, the peak absorption coefficient at the lowest tuned 281 Hz was as high as 0.93. Such distributed tunability based on a fault-tolerant electro-adhesive membrane will also be applicable to other acoustic meta-materials that embody the components of micro-perforated or micro-slit panels.

** The authors are grateful to Singapore Asahi Chemical and Solder Industries Pte. Ltd. for supplying the silver-nanowire filled conductive polymer ink.

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