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Acoustic bianisotropy in a subdivided meta-layer.

Carl R Hart1, Michael B Muhlestein1, Cody M Best1

  • 1U.S. Army Engineer Research Development Center, Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire 03755, USA.

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This study explores resonant Willis meta-layers, demonstrating tunable acoustic absorption. By manipulating internal asymmetry, researchers achieved directionally dependent absorption across multiple frequency bands.

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Area of Science:

  • Acoustics
  • Metamaterials
  • Solid Mechanics

Background:

  • Willis materials exhibit momentum-strain coupling due to microscopic asymmetry and mesoscale nonlocal effects, leading to acoustic bianisotropy.
  • Resonant Willis meta-layers utilize incorporated resonators to achieve Willis coupling within specific frequency bands.
  • Subdividing meta-layers into multiple resonator shapes enables Willis coupling across multiple frequency bands, with self-similar shapes leading to power-law dependent resonance frequencies.

Purpose of the Study:

  • To measure and model resonant Willis meta-layers with parallel resonators.
  • To investigate the tuning of Willis coupling through manipulation of internal asymmetry.
  • To observe and analyze directionally dependent acoustic absorption and its tunability.

Main Methods:

  • Measurements were conducted using a transmission impedance tube.
  • Lumped-element analysis was employed for theoretical predictions.
  • Resonator shapes were derived from subdivisions of the equilateral triangle.

Main Results:

  • Willis coupling was achieved over several frequency bands between 500-1500 Hz.
  • Tuning of Willis coupling was demonstrated by altering resonator neck asymmetry.
  • Directionally dependent acoustic absorption was observed and tunable via internal asymmetry non-uniformity.

Conclusions:

  • Resonant Willis meta-layers offer a platform for achieving tunable, directionally dependent acoustic absorption.
  • Internal asymmetry is a key factor in controlling Willis coupling and absorption characteristics.
  • The findings have implications for designing advanced acoustic metamaterials.