Passive Acoustic Metamaterials for Low Frequencies—Theories, Types, Testing, and Future Directions

Metamaterials, engineered to exhibit unique properties, not found in natural materials, are a key focus of modern scientific research. Acoustic metamaterials are designed to manipulate or attenuate acoustic waves. Early designs of acoustic metamaterials show promising results in attenuating sound waves in medium and high-frequency ranges but lack effectiveness for low frequencies. In recent years, there has been a shift toward the research of passive acoustic metamaterials, designed for low frequencies, with a trend toward additive manufacturing for ease of fabrication. Over 45 acoustic design theories have been reviewed, along with 112 low-frequency acoustic metamaterials designed in the last 5 years. This comprehensive review ensures the validity and reliability of present findings and equips the knowledge to select the most appropriate acoustic theory, metamaterial type, and testing standards for analyzing metamaterial. The article also discusses computational methods in the design process and compares various existing metamaterial designs and their applications in areas like environmental noise reduction, sound isolation, and other health-related applications. Finally, it reviews testing methods for experimental verification of acoustic metamaterials. This article aims to steer the future course of metamaterial science by integrating a wide range of research, instilling confidence in the validity and reliability of present findings.