Forget metamaterial: It does not improve sound absorption performance as it claims

The term `sub-wavelength' is commonly used to describe innovative sound-absorbing structures usually labeled as `metamaterials'. Such structures, however, inherently do not bring groundbreaking advancements. This study addresses the limitations imposed by the thickness criterion of Yang et al. by introducing the concept of equivalent mass-spring-damping parameters within the resonator framework. This innovative approach introduces an index of `half-absorption bandwidth' to effectively overcome the thickness restriction. Four practical cases are then presented to correct prevalent misleading conceptions about low-frequency, broadband absorption as claimed. The phenomenon of mass disappearing in the expression of sound absorption coefficient supports the conclusion that volume is the only determinant factor in sound absorption performance. Any attempts to improve sound absorption solely through geometry and structural designs would inevitably sacrifice the half-absorption bandwidth. Additionally, the concept of negative stiffness or bulk modulus is merely a mathematical convention without any real improvement in absorption performance. Overall, this research focuses on the physical mechanism of sound-absorbing structures by correcting traditional misunderstandings, and offers a comprehensive framework for assessing and enhancing sound absorption.