Acoustic Metamaterials: Air Permeable Super Sound Attenuators and Acoustic Metalenses

Abstract

In recent decades, acoustic metamaterials have become more practical for diverse applications of sound manipulation, rather than academic curiosities and theoretical derivations. This kind of artificial material demonstrates revolutionary functionalities beyond the limitations of natural material properties, such as efficiently attenuating sound without high density and bulky size, transmitting sound in a highly concentrated and magnified manner, etc. Owing to these fantastic merits, acoustic metamaterials exhibit the feasibilities to present solutions that conventional materials do not possess. For instance, a large number of MEMS sensors and actuators are notably susceptible to the resonance caused by high power acoustic noise, but the accumulated heat emitted from those devices causes functional failures and physical damages as well. In this case, conventional packaging materials are inadequate to solve both problems at the same time. As another example, ultra compact structures typically only dissipate or diffuse acoustic energy. In this dissertation, two super sound attenuators and two innovative acoustic metalenses are proposed by taking advantage of 3D printing techniques. These metamaterials were designed as solutions to these two problems, respectively. For the first sound attenuator, a micro scale open-through dual expansion chamber (ODEC) package employs both acoustic resonance and thermo-acoustic effects to achieve high and continuous attenuations from 2000 to 8200 Hz. Corresponding experiments with 18 configurations of the ODECs and related control group samples (non-ODEC) were conducted to validate the theoretical predictions with respect to transmission loss (TL). The geometric effect of the TL is also investigated with respect to chamber length and radius. The ODECs perform in the manner of low-pass filters, and their corresponding highest transmissions range from 28.87 to 44.51 dB at 8100 Hz. For the second sound attenuator, an air permeable labyrinth element (LE) acoustic metamaterial exhibits low transmission, low reflection, and high absorption. This is theoretically expected from the complex open through structure at a deep sub-wavelength scale to provide low sound transmissions under 500 Hz, which is air permeable and insusceptible to circumambient changes. Experimental results and simulations meet the expectations of high absorption, a substantial near unity absorption from 223 to 327 Hz, and experimentally exhibit impedance matching in this broad low-frequency range. For the first metalens, a flat Fresnel lens was designed and tested. This lens demonstrated outstanding sound beams from 8120 to 8270 Hz, and the longest beam could reach as far as 65 cm at 8219 Hz with 15 cm width of the beam. For the second metalens, a perforated sinusoidal channel array with sub-wavelength distances was designed and tested. Enhanced directional far-field sound transmission over a large frequency range was realized through using this compact metalens. Numerical simulations and experimental tests demonstrated a pronounced diffraction-limited sound beam from 7390 to 7600 Hz, which reached as far as 57.6 cm at 7551 Hz with a width of 9 cm. The conceptual and experimental work of these four compact air permeable sound metamaterials demonstrate the outstanding performance of sound absorption and transmissive sound focusing with wide frequency bands, which show the potential abilities for sound manipulations in heavy industry, aerospace, pharmacy, medicine, ocean applications, etc.