An ultrathin underwater acoustic absorbing metamaterial by inverse design

The thickness of a metamaterial reflects its ability to regulate sound waves and vibrations. The study proposes an ultrathin underwater acoustic absorbing metamaterial using a topology optimization method for reverse design. The optimized metamaterial obtains the capability of extraordinary acoustic absorption in the low-frequency case, achieving an absorption coefficient of 70% for the waves with a wavelength of 100 times thickness. With the combined effect of cavity resonance and wave mode transformation, the metamaterial achieves an average absorption coefficient of more than 90% for the acoustic wave with wavelength 7.5 to 100 times thickness. The research significantly advances the practical application of underwater acoustic metamaterials.