Adaptive metamaterials for broadband sound absorption at low frequencies

We propose and design a new adaptive sound absorption metamaterial targeting broadband airborne noise at extremely low frequencies. The metamaterial consists of two piezoelectric smart elements: a circular aluminum membrane with surface-bonded piezoelectric films controlled by shunting circuits enclosed with an air cavity for nearly total acoustic absorption at narrow-band frequencies; a hybrid-circuit shunted piezoelectric stack which is mechanically grounded attached to the center of the membrane for purely stiffness control to broaden this high-absorption bandwidth. A piezoelectric-structural-acoustic coupled model is firstly developed to evaluate the sound absorption of the metamaterial. We then perform analytical and numerical tests on metamaterials with and without the piezoelectric stack to design a metamaterial with broadband absorption at desired low frequencies. The underlying adaptive mechanism is to automatically regulate the effective acoustic resistance and reactance of the metamaterial to achieve impedance match conditions, according to different frequencies of inputs. Our numerical results demonstrate that the absorption coefficient of the adaptive metamaterial can be greater than 0.9 in the frequency region, 112-236 Hz with the relative bandwidth being around 0.7. The metamaterial thickness is 30 mm, which is nearly 1/65.6 wavelength of the central frequency of the absorption band. The proposed adaptive metamaterial may open a new avenue towards broadband sound absorption at extremely low frequencies.