Controlling the propagation of sound has long been a desirable goal in science and engineering. The emergence of acoustic metamaterials, which are artificial structures composed of meta-atoms, offers unprecedented capabilities in acoustic wave engineering. These metamaterials are designed based on their constitutive unit cells and behave like a continuous material with unconventional properties in the bulk. Over the past two decades, research on acoustic metamaterials has demonstrated a plethora of fascinating phenomena ranging from negative refractive indices to acoustic cloaking (Liu et al., 2000; Lee et al., 2010; Liang and Li, 2012; Cummer et al., 2016; Liao et al., 2021).

In recent years, the community has witnessed strong growth in the development of two-dimensional (2D) equivalent acoustic metamaterials. These structures, termed acoustic metasurfaces (AMs), are capable of providing non-trivial local phase shifts, amplitude modulation, and extraordinary sound absorption. Thanks to their thinness which is usually in the sub-wavelength, AMs have added value and functionalities in comparison with other acoustic metamaterials with small footprint (Xie et al., 2014; Cheng et al., 2015; Zhao et al., 2017; Assouar et al., 2018; Quan and Alu, 2019). Numerous exotic acoustic phenomena such as sound cloaking (Faure et al., 2016; Ma et al., 2019; Fan et al., 2020; Zhou et al., 2020), sound splitting (Zhai et al., 2018; Ding et al., 2021), sound absorption (Ma et al., 2014; Song et al., 2019; Liu et al., 2021; Li et al., 2022a; Guo et al., 2022), anomalous reflection or refraction (Diaz-Rubio and Tretyakov, 2017; Li et al., 2019a; Zhu and Lau, 2019; Li et al., 2020a; Chiang et al., 2020; Song et al., 2021), sound focusing (Zhu et al., 2016a; Lombard et al., 2022), one-way sound propagation (Zhu et al., 2015; Jiang et al., 2016), and medical ultrasound (Tian et al., 2017; Hu et al., 2022) have been proposed and demonstrated using AMs. AMs possess unusual features, including selective focusing and negative refraction, are enabled by the generalized Snell’s law, which adds a new degree of freedom to control the behavior of transmitted or reflected waves by incorporating a lateral momentum (Yu et al., 2011) (see Figure 1). In this approach, conventional AMs modulate the phase change inside the unit cells by changing the effective spatial path of waves via space-coiling structures or by generating a phase delay through resonance.

While these AMs provide unparalleled potential to control acoustic waves, they generally have fixed configurations and cannot be easily altered. As such, research efforts have been devoted to make them tunable, reconfigurable, and programmable to further expand their capacities. Tunability adds another degree of freedom to control the sound transmission like tuning the dispersion or frequency, wavenumber, and topological states of acoustic waves (Tian et al., 2020). Incorporating tunable structures that can operate at different frequencies can also increase the bandwidth of the devices (Peng et al., 2022). There are a few approaches to achieve the desired tunable control of AMs. For example, origami and kirigami, the art of paper folding and cutting, can increase the ultimate strain on the sheets and applications have produced aerodynamic benefits through wing surface texturing (Gamble et al., 2020). These principles have been utilized in AM design by introducing reconfigurable geometries.

The approach of geometrical tuning can be also exemplified by asymmetric structures via rotation. For instance, rotatable anisotropic three-component resonators have been proposed to induce non-degenerate dipolar resonance, which further leads to an evident phase change in low frequencies (Li et al., 2020b). Active designs where electroacoustic transducers and circuits are used, on the other hand, represent another efficient way to realize tunable AMs (Li et al., 2022b). As an example, this has been implemented in acoustic cloaking which enables sound to flow around an obstacle without the trace of reflections using programmable AMs (Li et al., 2019b). The ongoing development of tunable, reconfigurable, and programmable AMs has opened up new possibilities for controlling the behavior of sound in different acoustic media.

This review aims to provide insights into the recent development of tunable, reconfigurable, and programmable AMs. While these notations have all been used extensively and sometimes interchangeably in the literature, a tunable AM should refer to a structure that exhibits some form of tunability. In contrast, reconfigurable and programmable AMs usually denote metasurfaces with more advanced controllability in which the functionality or application can be tailored to meet specific requirements. Here, the main approaches to achieve tunable control are summarized and divided into three main categories: mechanical tuning, active control, and field-responsive material. The principles and mechanism for these approaches are reviewed, and demonstrations of typical applications are presented. At the end, we point out a few pressing questions in the current research and provide an outlook for growth in this burgeoning field.

To perform certain functions, AMs typically have a fixed microstructure that can regulate the local scattering of acoustic waves. As such, mechanical tuning manifests itself as a straightforward way to provide tunable functionality by leveraging geometric changes of the microstructures or the overall profile of the metasurface. When propagating through or interacting with these tunable AMs, different phase delays or amplitudes can be achieved, which together represent the tunability of the AMs.

Actively controlled metasurfaces have taken center stage in recent years since they provide elegant ways to effectively control and manipulate AMs after fabrication. In comparison with passive AMs that have a fixed functionality once fabricated, the implementation of active elements affords great flexibility and convenience in tuning their responses with reconfigurable electronic circuits. The most common application of such an approach is in membrane-type AMs where, depending on their type and nature, properties such as surface tension can be easily tuned by applying an external voltage (Cheng et al., 2018; Shen et al., 2019; Zhai et al., 2019; Akl and Baz, 2021; Zhang et al., 2021; Kovacevich and Popa, 2022; Peng et al., 2022). Zhang et al. (2016), proposed smart AM designs which exhibit wave reflection at low frequencies with high efficiency (See Figure 4A). As presented in Figure 4B, a tunable AM was proposed in which composite sheets of PVDF membrane and polyimide tape were inserted into the cavity (Li et al., 2022c). Some studies showed that by using a direct voltage applied to the membrane, the eigen-frequencies of AMs could be tuned, and therefore, the wave transmission phase could be tuned as well (Kumar and Lee, 2019). In the case of an alternating voltage applied to the membrane, the vibrations of the membrane AMs resulting from such excitation could be greatly enhanced or suppressed, which could help achieve a variety of functionalities such as acoustic switches with high on/off ratios and phase modulations (Xiao et al., 2015), changing the effective modulus (Li et al., 2019c), as an anomalous acoustic reflector (Zhai et al., 2021), for steerable reflections over a wide frequency band (Lissek et al., 2018), and for asymmetric transmission (Geib et al., 2021; Zhou and Baz, 2023).

These active approaches have been applied to the design and realization of reconfigurable AMs. For example, Popa et al. (2015) proposed a metamaterial slab in which the metamaterials can play multiple roles simultaneously and have application in non-linear imaging applications (Figure 4C).Tang et al. (2019) attempted to create a versatile wave-front manipulation device by designing an ultrathin membrane-type active acoustic metasurface. The local sound intensity could be actively reconfigured by dc voltage control of electromagnets attached to the membrane-type metamaterial. This concept has been used to perform spatial sound modulation to harness the degrees of freedom that underlie reverberating sound by combining metamaterials and wave-field shaping (Figure 4D) (Ma et al., 2018). Utilizing an adaptive AM approach, a noise shielding device was designed by Steiger et al. (2016) (Figure 4E) to compensate for the acoustic transmission loss of the sound transmitted through adaptive AMs. Also, the precise electronic feedback control of values was presented to overcome capacitance matching with a fixed adjustment of parameters. Sound barriers can also be designed by utilizing acoustic bianisotropic materials which contain non-zero strain-to-momentum coupling by leveraging active components. Basically, compact active meta-atoms that display local responses to external sound can help achieve better properties in terms of bandwidth, attenuation, and shielded volume (Popa et al., 2018).

Apart from mechanical and electrical tuning, another method of achieving programmable AMs is by using field-responsive materials. In this approach, either the property or the shape of these materials changes when an external field is applied, and wave–matter interactions are changed in ways that correspond to the property or shape changes, which produces a tunable or programmable metasurface. In general, the use of magnetoactive materials and temperature changes has been proposed as a solution. These have conspicuous advantages like low-frequency wave manipulation (Chen et al., 2014; Chen et al., 2017; Liu et al., 2020), switching on and off of topological states (Lee et al., 2022), and production of negative modulus and density (Xia et al., 2016) which leads to the acquisition of a wider acoustic band, better adjustability, and more balanced performance.

The objective of this review was to provide fresh insights into the fundamental concepts and applications of tunable, reconfigurable, and programmable AMs. To this end, recent advancements achieved in the design and development of AMs were highlighted. Main strategies to achieve tunable, reconfigurable, and programmable AMs were summarized in three main categories: mechanical tuning, active control, and field-responsive materials. In the past decade, AMs have emerged as a new generation of acoustic metamaterials due to their functionalities and mechanical properties. Although much progress has been made in AMs, some promising directions have yet to be fully explored because of technological gaps between large-scale practical applications and lab-scale research. For instance, mechanical tuning provides a reliable means to achieve the required reconfigurable wave control effects by leveraging the geometrical change of the structures. However, this approach typically allows only a relatively slow tuning rate. This poses challenges to apply the approach to applications such as communication in which dynamic tuning is desired. A large degree of change in geometry may also prevent its application in a confined space.

The implementation of active elements affords great flexibility and convenience in being able to quickly regulate their response by fast electronic circuits with high tunability. Despite the great convenience offered by active control, the requirement of conversions between acoustic and electrical signals as well as the relatively complicated feed circuity can still create a bottleneck for many practical applications. Therefore, there is a critical need for further engineering developments in simple yet efficient circuits and electroacoustic systems for a variety of functionalities. Field-responsive materials can adjust and control AMs in a non-contact and reconfigurable manner, which could help solve some of the existing issues requiring sophisticated tuning mechanisms. Nevertheless, their usage is still limited at this moment and more work needs to be carried out in this direction to make them applicable to a real-world setup. For example, they use very specific materials that can induce acoustic interactions and also need the excitation of an external field which could also limit their broad application. While it is evident that tunable and programmable AMs are a relatively new field and are promising in acoustic wave manipulation, more research needs to be carried out, especially in realizing real-time reconfigurable properties with simple tuning mechanisms. It is hoped that both conceptual research and rapid development will be carried on to increase their application in industry.