Tunable two-dimensional acoustic meta-structure composed of funnel-shaped unit cells with multi-band negative acoustic property

Abstract

This paper presents a two-dimensional heat-exhaust and sound-proof acoustic meta-structure exhibiting tunable multi-band negative effective mass density. The meta-structure was composed of periodic funnel-shaped units in a square lattice. Each unit cell operates simultaneously as a Helmholtz resonator (HR) and an extended pipe chamber resonator (EPCR), leading to a negative effective mass density creating bandgaps for incident sound energy dissipation without transmission. This structure allowed large heat-flow through the cross-sectional area of the extended pipe since the resonance was generated by acoustic elements without using solid membranes. The pipes were horizontally directed to a flow source to enable small flow resistance for cooling. Measurements of the sound transmission were performed using a two-load, four-microphone method for a unit cell and small reverberation chamber for two-dimensional panel to characterize the acoustic performance. The effective mass density showed significant frequency dependent variation exhibiting negative values at the specific bandgaps, while the effective bulk modulus was not affected by the resonator. Theoretical models incorporating local resonances in the multiple resonator units were proposed to analyze the noise reduction mechanism. The acoustic meta-structure parameters to create broader frequency bandgaps were investigated using the theoretical model. The negative effective mass density was calculated to investigate the creation of the bandgaps. The effects of design parameters such as length, cross-sectional area, and volume of the HR; length and cross-sectional area of the EPCR were analyzed. To maximize the frequency band gap, the suggested acoustic meta-structure panel, small neck length, and cross-sectional area of the HR, large EPCR length was advantageous. The bandgaps became broader when the two resonant frequencies were similar.

Model for analytical investigation on meta-lattice truss for low-frequency spatial wave manipulation

“…The bandgap, which is the specific range of frequencies where propagation of an applied wave is stopped, is the most crucial features of metamaterials [26]. Therefore, much research effort primarily contributed to metamaterials' fundamental mechanism with an attempt to seek approaches to broaden the bandgap of metamaterials or make it tunable [27], [28], [29].To investigate the relation between the effective dynamic mass density and the oscillation frequency, Milton and Willis [30] proposed a rigorous model of metamaterials utilizing the typical motion equations for a rigid bar and Newton's second law to simulate the dynamic effective mass density as a function of the resonant frequency. The single mass-in-mass model was originally introduced by Huang and Sun [10] offering the negativity of mass property over a specific frequency range and this model was applied to lattice systems to broaden the bandgap by Liu et al [31].…”

3D Hilbert fractal acoustic metamaterials: low-frequency and multi-band sound insulation

“…One of the most impressive features is the sound forbidden-band ability with Bragg-scattering and/ or local resonance to achieve low-frequency sound attenuation [5,13]. Additionally, sound absorber and attenuator possessing negative dynamic effective parameters [14][15][16], like including membrane-type [17,18], Helmholtz resonators [19,20], and labyrinthine structures [21][22][23][24][25], always play a prominent role at the low-frequency sound propagation. However, most acoustic metamaterials have only one narrowband since the resonator in a specific configuration usually possesses one resonant mode [20].…”

Investigation on dynamic effective parameters of perforated thin-plate acoustic metamaterials

“…In fact, for application to refrigerators whose noise is produced by rotating machine parts, an acoustic device needs to provide sound reduction without significant resistance to airflow generated by internal fans [34]. Therefore, acoustic metamaterials with holes will solve the problem.…”

“…The location of the forbidden band and its band width can be engineered by changing the period of the array hole and the thickness of the gap layer placed between the two holey plates [37]. In addition, Cho et al presented a two-dimensional acoustic meta-structure composed of funnel-shaped unit cells with multi-band negative acoustic property, which could be used as a heat-exhaust and soundproof acoustic metamaterials [34]. Kim et al presented an air transparent soundproof window that consists of a 3D array of strong diffraction-type resonators with many holes centered on each individual resonator [38].…”