H.M. Dai

Bio: H.M. Dai is an academic researcher. The author has contributed to research in topics: Metamaterial & Absorption (acoustics). The author has an hindex of 2, co-authored 2 publications receiving 390 citations.

Acoustic metamaterial panels for sound attenuation in the 50–1000 Hz regime

Abstract: We show experimentally that thin membrane-type acoustic metamaterials can serve as a total reflection nodal surface at certain frequencies. The small decay length of the evanescent waves at these frequencies implies that several membrane panels can be stacked to achieve broad-frequency effectiveness. We report the realization of acoustic metamaterial panels with thickness ≤15 mm and weight ≤3 kg/m2 demonstrating 19.5 dB of internal sound transmission loss (STL) at around 200 Hz, and stacked panels with thickness ≤60 mm and weight ≤15 kg/m2 demonstrating an average STL of >40 dB over a broad range from 50 to 1000 Hz.

Non-periodic Locally Resonant Sonic Materials

Abstract: Locally resonant sonic materials (LRSMs) are a new class of meta-materials that can block sound waves well beyond the limit of the conventional mass density law. The LRSMs reported so far are made of local resonators arranged periodically in a matrix, although the period is usually much smaller (< 1/20) than the wavelength in the matrix material. At resonance, the effective wavelength becomes much smaller, and the interference from the individual resonators could become significant. Here we report further study of the properties of LRSMs. We found that in some deliberately structured LRSMs the main acoustic properties is determined only by the resonators, independent of their spatial arrangement. In particular, the acoustic properties of a single resonator resemble that of an assembly of the resonators. The lateral coupling of resonators, when deliberately introduced, generates multimode resonances, leading to multiple stop bands in transmissions. The dependence of the resonant frequency f on the mass of the weight M in the single-weight resonator follows the simple mass-and-string relation, namely 1/ f M ∝ . The large phase shift associated with the transmission peak that accompanies the stop band provides an approach for designing zoneplates to focus low frequency sound waves, in much the same way as optical zoneplates. Simple estimations show that effective focusing could be achieved for a plate of the diameter of one wavelength and phase shift difference of less than 110 degrees across different zones.

Controlling sound with acoustic metamaterials

Abstract: Acoustic metamaterials can manipulate and control sound waves in ways that are not possible in conventional materials. Metamaterials with zero, or even negative, refractive index for sound offer new possibilities for acoustic imaging and for the control of sound at subwavelength scales. The combination of transformation acoustics theory and highly anisotropic acoustic metamaterials enables precise control over the deformation of sound fields, which can be used, for example, to hide or cloak objects from incident acoustic energy. Active acoustic metamaterials use external control to create effective material properties that are not possible with passive structures and have led to the development of dynamically reconfigurable, loss-compensating and parity–time-symmetric materials for sound manipulation. Challenges remain, including the development of efficient techniques for fabricating large-scale metamaterial structures and converting laboratory experiments into useful devices. In this Review, we outline the designs and properties of materials with unusual acoustic parameters (for example, negative refractive index), discuss examples of extreme manipulation of sound and, finally, provide an overview of future directions in the field. Acoustic metamaterials can be used manipulate sound waves with a high degree of control. Their applications include acoustic imaging and cloaking. This Review outlines the designs and properties of these materials, discussing transformation acoustics theory, anisotropic materials and active acoustic metamaterials.

Acoustic metamaterials: From local resonances to broad horizons.

Abstract: Within a time span of 15 years, acoustic metamaterials have emerged from academic curiosity to become an active field driven by scientific discoveries and diverse application potentials. This review traces the development of acoustic metamaterials from the initial findings of mass density and bulk modulus frequency dispersions in locally resonant structures to the diverse functionalities afforded by the perspective of negative constitutive parameter values, and their implications for acoustic wave behaviors. We survey the more recent developments, which include compact phase manipulation structures, superabsorption, and actively controllable metamaterials as well as the new directions on acoustic wave transport in moving fluid, elastic, and mechanical metamaterials, graphene-inspired metamaterials, and structures whose characteristics are best delineated by non-Hermitian Hamiltonians. Many of the novel acoustic metamaterial structures have transcended the original definition of metamaterials as arising from the collective manifestations of constituent resonating units, but they continue to extend wave manipulation functionalities beyond those found in nature.

Dark acoustic metamaterials as super absorbers for low-frequency sound

Abstract: The attenuation of low-frequency sound has been a challenging task because the intrinsic dissipation of materials is inherently weak in this regime. Here we present a thin-film acoustic metamaterial, comprising an elastic membrane decorated with asymmetric rigid platelets that aims to totally absorb low-frequency airborne sound at selective resonance frequencies ranging from 100-1,000 Hz. Our samples can reach almost unity absorption at frequencies where the relevant sound wavelength in air is three orders of magnitude larger than the membrane thickness. At resonances, the flapping motion of the rigid platelets leads naturally to large elastic curvature energy density at their perimeter regions. As the flapping motions couple only minimally to the radiation modes, the overall energy density in the membrane can be two-to-three orders of magnitude larger than the incident wave energy density at low frequencies, forming in essence an open cavity.

Acoustic metasurface with hybrid resonances

Abstract: Acoustic impedance-matched surfaces do not reflect incident waves. Traditional means of acoustic absorption have so far resulted in imperfect impedance matching and bulky structures, or require costly and sophisticated electrical design. Inspired by electromagnetic metamaterials, a subwavelength acoustically reflecting surface with hybrid resonances and impedance-matched to airborne sound at tunable frequencies is now demonstrated.