Ultra-Low-Frequency Broadband Sound Absorption Characteristics of an Acoustic Metasurface with Pie-Sliced Unit Cells

In this work, an acoustic metasurface composed of pie-sliced resonator segments is proposed and its attenuation characteristics in ultra-low-frequency regime are examined using analytical, numerical, and experimental means. This metasurface absorber is systematically designed and realized with inhomogeneous unit cells in the shape of sliced pie to accomplish ultra-low-frequency (<100 Hz) noise cancellation. Firstly, a sound absorber with four unit cells having deep sub-wavelength thickness (\(\lambda \)/39) is fabricated and it exhibited a sound wave mitigation of more than 80% throughout the low-frequency range of 212–276 Hz. For enhancing the broadband absorption in the ultra-low-frequency regime, an acoustic absorber with eight pie-sliced unit cells is designed, realized and tested. Thereafter, a compact absorber with very-low vertical (\(\lambda /34\)) and lateral (\(\lambda /52\)) dimensions is considered and it accomplished more than 85% sound absorption in the ultra-low-frequency domain of 66–100 Hz. It is further demonstrated that, by tailoring the geometrical features such as cavity depth, resonator diameter, neck length, sector angle and number of unit cells of the metastructure, the bandwidth and frequency of the absorption peak can be modified as per the requirement. Owing to the compactness, ease of production and high absorption capacity over a wide band in the ultra-low-frequency regime, the introduced acoustic metasurface is a potential candidate for noise control applications.