Metamaterials are sub-wavelength structures having characteristics that are not observed naturally. Acoustic metamaterials provide remarkable functionalities such as absorption, cloaking, beam focusing, and sub-wavelength imaging [1]. The sound waves can be controlled and manipulated by incorporating Helmholtz resonators, quarter-wave resonators, membranes, sub-wavelength perforations or slits, and sonic crystals in a material structure [2]. These have recently attracted noticeable attention due to their lightweight and compact dimensions.
Designing broad-band absorbers at the longer wavelengths and beam manipulators for various applications is a challenge. To dissipate low-frequency sound with conventional materials is impractical in many applications due to their bulky size and weight. Low-frequency sound attenuation in subwavelength space is an interesting and challenging task [3]. Beam formation is very important for achieving larger depth of focusing and reducing the divergence. For example, Bessel-like beams have self- healing property apart from large depth of focus [4]. A metamaterial/phononic crystal-based approach is proposed to address the challenges. Current and future investigations and experiments on beam focusing designs and compact broad-band absorber designs would be discussed.

References:
[1] Fok, Lee, Muralidhar Ambati, and Xiang Zhang. "Acoustic metamaterials." MRS bulletin 33.10 (2008): 931-934.
[2] Cummer, Steven A., Johan Christensen, and Andrea Alu. "Controlling sound with acoustic metamaterials." Nature Reviews Materials 1.3 (2016): 1-13.
[3] Gao, Nansha, et al. "Acoustic metamaterials for noise reduction: a review." Advanced Materials Technologies (2022): 2100698.
[4] Jimenez, Noe, et al. "Acoustic Bessel-like beam formation by an axisymmetric grating." Europhysics Letters 106.2 (2014): 24005