Perfect acoustic absorption of Helmholtz resonators via tapered necks

A neck tapered Helmholtz resonator possessing brilliant sound absorption in low-frequency is designed and demonstrated in this work. The strategy offers an extra geometrical degree of freedom, which will benefit perfect absorption. The structure can not only achieve perfect-absorption at 132 Hz across a deep subwavelength thickness of lambda/50, but also realize adjustable perfect-absorption from 132 to 236 Hz with the same external shape. The effects of acoustic impedance and structural parameters have been comprehensively discussed for illustrating the structural advantages of sound absorption in low-frequency. Our results can pave a way for designing high-efficiency acoustic devices for sound absorption and impedance engineering.