We introduce a multi-coiled acoustic metasurface providing a quasi-perfect absorption (reaching 99.99% in experiments) at extremely low-frequency of 50 Hz, and simultaneously featuring an ultrathin thickness down to λ/527 (1.3 cm). In contrast to the state of the art, this original conceived multi-coiled metasurface offers additional degrees of freedom capable to tune the acoustic impedance effectively without increasing the total thickness. We provide analytical derivation, numerical simulation and experimental demonstrations for this unique absorber concept, and discuss its physical mechanism which breaks the quarter-wavelength resonator theory. Furthermore, based on the same conceptual approach, we propose a broadband lowfrequency metasurface absorber by coupling unit cells exhibiting different properties.Due to the weak intrinsic dissipation of conventional materials in the low-frequency region, the perfect absorption of sound at low frequency (<100Hz) is still a scientific challenge [1]. To enhance the dissipation, it is necessary to increase the energy density inside the relevant material by some means like through resonances [2]. In the past two decades, the explosion of interest in developing artificial resonant structures like acoustic metamaterials [3][4][5][6] and metasurfaces [7][8][9][10][11][12] was drastically increased owing to their exotic capabilities of manipulating sound waves and their deep subwavelength thickness [13, 14]. The remarkable characteristics of acoustic metamaterials such as negative mass density, refractive index, double negativity, and controlled anisotropy have attracted massive research in the field of developing deep subwavelength sized acoustic devices for low-frequency applications such as negative refraction [13], deep subwavelength focusing/imaging [14], cloaking [15], perfect absorbers with ultrathin thickness [12,[14][15][16][17][18][19][20][21][22] etc. In related researches, perfect metasurface absorbers have received considerable attention, and variety of acoustic metasurfaces designs have been designed for the applications in noise control at low-frequency regime, which could be used in aircraft, locomotives, automobiles, machines, and buildings [13,[17][18][19][20][21][22][23][24].Ultra-thin acoustic absorber metasurface is generally based on a single or hybrid resonant system [5-12; 16-23]. The hybrid resonant system [25] usually has a broader bandwidth than a single resonant one. One interesting way to design the perfect acoustic absorber is to use an ultrathin decorated membrane with thin air chamber. Such a system was reported by Ma et al. [7] to form hybrid resonance capable of to realizing complete absorption at low-frequency range [8, 9] with a thickness of about ~1/133 of the operating wavelength. However, this design requires

A severe limitation of current acoustic metasurfaces remains in their modest tunability to meet multi-frequency requirements and alterable functionalities on demand. Here, a reconfigurable curved acoustic metasurface for acoustic cloaking and illusion is reported. The structure is composed of an array of tunable helical units to break this limitation and realize continuously versatile sound manipulations. We theoretically, numerically and experimentally investigate the channel length represented by the helical depth, which is used to achieve full 2π phase shift continuously over the frequency range from 2 to 7 kHz. As pragmatic examples, we present by full-wave numerical simulations the concept of a curved metasurface for the continuously tunable acoustic multifunction, including broadband carpet cloaking and ground illusion at a wide working band. Then, we experimentally demonstrate these functionalities by showing an excellent effect to restore the disturbed reflective field from a cloaked object or to mimic an arbitrary shaped ground.