  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.
