Acoustic bianisotropy in a subdivided meta-layer

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.