Low frequency tuning of space-folded acoustic metamaterial amplifiers through effective density distribution method.

Space-folded acoustic metamaterials (SFAMs) have exceptional low frequency sound control capability in deep sub-wavelength scale, showing significant potential for weak acoustic signal detection. However, lowering the working frequencies of SFAMs may increase the device size, limiting their applications and raising production costs. To address these issues, this study proposes compact SFAM acoustic amplifiers operating in low frequencies, where their working frequencies are flexibly controlled by engineering effective density without increasing device size. Further experiments have demonstrated the capability of wide range frequency control of SFAM through engineering its effective density. The experimental results clearly indicate that the low frequency tuning of SFAMs results from controlling SFAMs' channel width gradients, which is associated with the spatial distribution of metamaterial's effective density. Further studies reveal that compared to the gradient density distribution method in SFAMs, the step density distribution method may have superior low frequency control of acoustic waves, which are capable of frequency tuning of SFAM acoustic amplifier over one octave band. This work provides possibilities for the development of low frequency acoustic amplifiers, where the merits include compact size, lightweight, and low cost, which are highly desirable for acoustics sensing and noise control applications.