Toward wideband steerable acoustic metasurfaces with arrays of active electroacoustic resonators

Abstract

We introduce an active concept for achieving acoustic metasurfaces with steerable reflection properties, effective over a wide frequency band. The proposed active acoustic metasurface consists of a surface array of subwavelength loudspeaker diaphragms, each with programmable individual active acoustic impedances allowing for local control over the different reflection phases over the metasurface. The active control framework used for controlling the reflection phase over the metasurface is derived from the Active Electroacoustic Resonator concept. Each unit-cell simply consists of a current-driven electrodynamic loudspeaker in a closed box, whose acoustic impedance at the diaphragm is judiciously adjusted by connecting an active electrical control circuit. The control is known to achieve a wide variety of acoustic impedances on a single loudspeaker diaphragm used as an acoustic resonator, with the possibility to shift its resonance frequency by more than one octave. This paper presents a methodology for designing such active metasurface elements. An experimental validation of the achieved individual reflection coefficients is presented, and full wave simulations present a few examples of achievable reflection properties, with a focus on the bandwidth of operation of the proposed control concept.

Acoustic metasurfaces

“…One candidate is an active material 115,116 (for example, a piezoelectric material), which is emerging as an important direction in the field of acoustic metamaterials 117 . Effective material responses that are not possible with passive materials are realized and can even be tuned by active materials 118,119 . Meanwhile, the realization of high-frequency acoustic metasurfaces calls for more sophisticated fabrication techniques.…”

Ultra-thin low frequency perfect sound absorber with high ratio of active area

“…This remark is not specific to this absorber but can be generalized to many resonant sub-wavelength absorbers: It is the damping that limits the sub-wavelength ratio in resonant absorbing devices that are build with three in series elements (mass, spring and damping) like the classical Helmholtz resonator. One possibility to overcome this limit is to add some gain in the system by supplying external energy using, for example, electro-dynamical devices [23,24], thermo-acoustic devices [25] or using the flow [26] which is present in many engineering applications.…”

PID-like active control strategy for electroacoustic resonators to design tunable single-degree-of-freedom sound absorbers

“…The apparent acoustic impedance of the AER can be adjusted accordingly following the desired frequency dependence, leading to a family of designs of Single-Degree-Of-Freedom (SDOF) and Multiple-Degrees-Of-Freedom (MDOF) sound absorbers [13,20,21,16]. Such tunability is the key in many applications, such as room mode damping [22,16], wavefront shaping [23] or aircraft engine tonal noise reduction [24]. Nevertheless, due to the unavoidable time delay in the control execution, as well as the inaccuracy in the parameter estimation required for control law definition, this type of feedforward-based control is always accompanied with a noticeable mismatch of the achieved acoustic impedance around the natural (passive) resonance of the ER, which could even make the controlled ER lost its passivity and produce instability.…”