A Compact Low-Frequency Acoustic Perfect Absorber Constructed with a Folded Slit.

Tunable perfect acoustic absorption at subwavelength thickness has been a prominent topic in scientific research and engineering applications. Although metamaterials such as labyrinthine metasurfaces and coiling-up-space metamaterials can achieve subwavelength low-frequency acoustic absorption, efficiently realizing tunable absorption under uniform and limited size conditions remains challenging. In this paper, we introduce a folded slit to enhance the micro-slit acoustic absorber, effectively improving its low-frequency acoustic absorption performance and successfully achieving a perfect acoustic absorption coefficient of 0.99 at a thickness of only 3.1 cm. By adjusting just two parameters of the folded area, we can efficiently achieve a tunable resonant frequency ranging from 525 to 673 Hz and a tunable acoustic absorption bandwidth of 56.5% to 60.2%, simultaneously maintaining uniform external dimensions. Additionally, the folded-slit absorber demonstrates a broader acoustic absorption bandwidth at lower frequencies, enhancing broadband absorption capabilities in the low-frequency domain. These results hold significant potential for the design of highly efficient, thin and tunable acoustic absorbers.