###### Abstract  
Acoustic metamaterials (AMs) are artificially structured materials composed of subwavelength units that enable acoustic phenomena not achievable with conventional materials and structures. This review defines and presents a distinct category referred to as soft acoustic metamaterials (SAMs), which use soft materials or reconfigurable structures to achieve enhanced acoustic functionality. These systems make use of the inherent flexibility of their materials or the deformability of their geometry to support passive, active, and adaptive functions. To capture this structural and functional diversity, we introduce a three-dimensional classification that considers geometry, acoustic control mechanisms, and functional responsiveness as interrelated aspects. The geometry is classified into two-dimensional metasurfaces and three-dimensional bulk structures. The control mechanisms include local resonance, phase modulation, attenuation, and structural reconfiguration. The response type refers to whether the system behaves passively, actively, or adaptively. Using this approach, we provide an overview of representative implementations and compare different design approaches to highlight their working principles and application areas. This review presents a structured classification for soft acoustic metamaterials and offers a foundation for future research, with broad potential in intelligent sound systems, wearable acoustics, and architectural applications. acoustic metamaterial; soft acoustic metamaterial; reconfigurable acoustic metamaterials  
structural deformability, adaptability to dynamic environments, and impedance matching [18]. Furthermore, fundamental physical constraints rooted in the causality of linear time-invariant systems place limits on the attainable bandwidth and thickness trade-offs in passive absorbers. In response to these limitations, recent studies have increasingly explored the use of soft and deformable materials for acoustic control [19; 20; 21]. Recent reviews of acoustic metamaterials have established classification frameworks based on tuning mechanisms (mechanical, electrical, etc. ), structural types (membrane, Helmholtz, etc. ), and operational modes such as passive and active operation. While these reviews illuminate specific aspects through their respective criteria, they face limitations in comprehensively capturing the complex characteristics of soft metamaterials, where multiple physical factors interact simultaneously. Table 1 summarizes representative prior reviews and their classification criteria, highlighting the scope and focus of each work. To address these limitations, this review introduces the concept of soft acoustic metamaterials (SAMs) as a distinct category that includes systems composed of soft materials or reconfigurable architectures. The term \"soft\" is adopted from the field of soft robotics, referring not only to material properties such as low-modulus elastomers and gels but also to the ability of the entire structure to deform and interact compliantly with external environments [22; 23]. In SAMs, such material or structural softness is not incidental but serves as a functional mechanism that enables acoustic manipulation by dynamically tuning key properties such as resonance frequency, wave path, and impedance. SAMs span a wide range of implementations, from entirely passive configurations that rely on intrinsic mechanical properties to systems that actively respond to external control inputs through the integration of actuators or sensors. Additionally, tunable systems that allow post-fabrication adjustment of properties using external stimuli such as static voltage or pre-applied strain combine features of both passive and active approaches while occupying a unique position in terms of operational flexibility. To provide a coherent classification of the growing diversity in SAM research, we organize these systems along three conceptually independent but complementary dimensions.