Abstract
Monopolar resonance is of fundamental importance in the acoustic field. Here, we present the realization of a monopolar resonance that goes beyond the concept of Helmholtz resonators. The balloon-like soft resonator (SR) oscillates omnidirectionally and radiates from all parts of its spherical surface, eliminating the need for a hard wall for the cavity and baffle effects. For airborne sound, such a low-modulus resonator can be made extremely lightweight. Deep subwavelength resonance is achieved when the SR is tuned by adjusting the shell thickness, benefiting from the large density contrast between the shell material and the encapsulated gas. The SR resonates with near-perfect monopole symmetry, as demonstrated by the theoretical and experimental results, which are in excellent agreement. For a lattice of SRs, a band gap occurs and blocks near-total transmission, and the effective bulk modulus exhibits a prominent negative band, while the effective mass density remains unchanged. Our study shows that the SR is suitable for building 3D acoustic metamaterials and provides a basis for constructing left-handed materials as a new means of creating a negative bulk modulus.
Highlights
Monopolar resonance is of fundamental importance in the acoustic field
Such artificially designed composites are known as acoustic metamaterials because they can break the conventional rules of sound wave propagation in ordinary materials[1,2,3,4,5,6,7,8,9]
We need to go beyond this conventional concept and achieve subwavelength monopolar resonance using a new approach, which is important to the development of acoustic metamaterials and is of general interest to the acoustic field
Summary
Monopolar resonance is of fundamental importance in the acoustic field. Here, we present the realization of a monopolar resonance that goes beyond the concept of Helmholtz resonators. The balloon-like soft resonator (SR) oscillates omnidirectionally and radiates from all parts of its spherical surface, eliminating the need for a hard wall for the cavity and baffle effects For airborne sound, such a low-modulus resonator can be made extremely lightweight. Built-in local resonance can endow composites that are made of subwavelength resonators with unusual dispersions, ranging from exponential decaying to negative refraction Such artificially designed composites are known as acoustic metamaterials because they can break the conventional rules of sound wave propagation in ordinary materials[1,2,3,4,5,6,7,8,9]. The study of acoustic metamaterials has been strongly stimulated by the advent of new types of resonators, such as the rubber coated sphere[1] and the mass-decorated membrane[2] These resonators are dipolar resonators and serve to realize negative mass density. Efforts have been pursued to realize monopolar resonance in a different manner from the classical HRs, in particular, the resonating unit formed by a couple of mass-decorated planar membranes and a solid ring[13]
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