Design of the Helmholtz resonators assembly for optimizing the broadband sound absorption of acoustic metasurface

The acoustic metasurface (AMS) can give a broadband sound absorption by a planar periodic array assembling small Helmholtz resonators tuned at different resonant frequencies, which work as unit cells which have equivalent small area to a fraction of the wavelength. The author has suggested that the broadband flat sound absorption is obtained by the AMS assembling the small Helmholtz resonators of more quantities at the lower resonant frequencies in the target frequency range. Their resonant frequencies are ordinary tuned by parameters of an airtight cavity with a multiple folded long neck reaching to the opening on the sound incident surface, which have a longer neck for a lower resonant frequency in this study. The formulation for the sound absorption coefficient of the AMS referring to the prior study shows the flat and nearly perfect sound absorption in one octave band with its center frequency of 125 Hz, 250 Hz and 500 Hz. Numerical case studies suggests that the area ratio of unit cells for the flat sound absorption of the AMS is formulated as a power function of frequency ratio to the center frequency and that its formulation will be identical for any center frequency.