Simplified design method for the broadband near-perfect sound absorption of the acoustic metasurface consisted with Helmholtz resonators

Broadband high sound absorption can be provided by the acoustic metasurface (AMS) consisted with planar periodic array of small Helmholtz resonators tuned at different resonant frequencies. The normal incident sound absorption coefficient of the AMS is formulated theoretically based on the normal incident acoustic impedance on the surface of unit cells, namely the Helmholtz resonators. This study provides the simplified design method for determine specifications, e.g., the opening size and neck length, the cavity volume of the unit cells, of which assembly gives the near-perfect sound absorption on the AMS at even low frequencies. Their predicted sound absorption characteristics are discussed with the measured normal incident sound absorption coefficient on test. AMS is additively manufactured with the fused deposition modeling (FDM) 3-D printer. Each of their unit cells has a multiple-folded long neck for tuning the resonant frequency at low frequency, even if the unit cell has the small total height. The measurement results and discussions conclude that the simplified design method gives the near-perfect sound absorption of the AMS with one-octave frequency bandwidth centered at 125, 250, or 500 Hz, and the method also have enough practical accuracy for predictions of the frequency characteristics.