On sound absorption mechanisms in relation to sub-wavelength and absorption bandwidth using mass-spring-damping model

Abstract

This study proposes a new approach based on equivalent mass–spring-damping parameters within a resonator framework, and introduces an index of ‘half-absorption bandwidth’. Four practical cases are then presented to correct prevalent misleading conceptions about low-frequency, broadband absorption as claimed. The disappearance of the mass term in the expression for the sound absorption coefficient reinforces the conclusion that volume is the key factor in sound absorption performance. This is because the mass and damping terms are governed by the neck, which is relatively small and easily achievable compared to the volume constraint. Any effort to enhance sound absorption purely through geometric optimization and parallel configuration will inevitably come at the cost of reducing the half-absorption bandwidth. Additionally, the notion of negative stiffness or bulk modulus in the frequency domain is simply a mathematical representation and does not inherently enhance absorption performance unless integrated with other bistable structures, such as magnetic or snap-through mechanisms. Overall, this research focuses on the physical mechanism of sound-absorbing structures, and offers a comprehensive framework for assessing and enhancing sound absorption.