Metamaterials are frequently defined as artificial materials engineered to have properties that have not been found in nature. Consisting of repeating patterns (a.k.a. unit cells) at scales much smaller than the wavelengths of interest, metamaterials provides a new way to manipulate waves through controlling the configuration (size, shape, orientations, etc.) of the unit cells. Similar to its optical counterpart, acoustic metamaterials (AMM) are designed to manipulate sound waves through tailoring its effective mass density and bulk modulus of the unit cells. In recent years, the concept of AMM has been extended beyond the scope of negative refraction materials and found in applications such as acoustic superresolution, acoustic cloaking based on transformation acoustics, zero-index medium, acoustic metasurface, and nonreciprocal acoustic devices.

Among the various types of AMM, membrane-based acoustic metamaterials have been reported to achieve 100% absorption, the acoustic analogue of photonic black-hole. However, the bandwidth is usually very narrow around some local resonance frequency, which limits its practical use. To address this limitation and achieve a broadband absorption, this paper first establishes a theoretical framework for unit cells of air-backed diaphragms, modeled as an equivalent mass-spring-dashpot system. Based on the impedance match principle, three different approaches are numerically investigated including tuning the cavity length, the static pressure in the cavity, and the effective damping of perforated plates. For a target working frequency, it is found that the bandwidth is inversely proportional to the effective mass when the resistance matches with air’s acoustic impedance.

As a proof-of-concept, prototypes with circular clamped polyimide diaphragms (diameter of 19 mm and thickness of 2 mil) and 3D printed substrates are fabricated and characterized using an acoustic impedance tube. The diaphragm is preheated in an oven to a higher temperature while being glued to a stainless-steel washer. In-plane tension is generated when the diaphragm cools down to the room temperature. When the metamaterial consists of uniform unit cells, acoustic absorption spectra are obtained for unit cells with different cavity lengths, which modulates the effective resistance (i.e., damping) of the air-backed diaphragm. Preliminary experiments show the feasibility to achieve an absorption bandwidth of 200-300 Hz at center frequency of 1.2–1.4 kHz. The bandwidth is further increased by combining three different unit cells, each having different in-plane tension.

This work not only establishes a framework for obtaining perfect absorption using acoustic metamaterial, but pays the way for developing a sub-wavelength light weight broadband acoustic absorber for a variety of applications in noise control.


Acoustic Metamaterial With Air-Backed Diaphragm for Broadband Absorption: A Preliminary Study

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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