Frequency-independent Sound Absorbing Metamaterials

Inspired by the concept of frequency-independent antennas in the electromagnetics, which theoretically exhibit a frequency-independent bandwidth, this paper extends the concept to acoustics by adopting a log-spiral and its modified shape to introduce a class of metamaterials called frequency-independent sound-absorbing metamaterials. Without the requirement of multiple resonators, these metamaterials achieve an ultra-broadband absorption spanning from 550 Hz for a wide range of incident angles up to 80°, using a single unit cell structure owing to Fabry–Pérot resonance, viscous boundary losses, and thermoviscous loss of the foam. Numerical and experimental analyses are provided to demonstrate the working principles of the proposed structures. Their performance is compared with archimedean spiral, and the influence of air and porous material parameters on the performance of the metamaterial is discussed in detail for normal and oblique incident waves. The underlying principle is discussed in detail, highlighting the distinctions between the proposed structures and state-of-the-art broadband metamaterials. The proposed structures represent a pioneering advancement in the field by offering compact and ultra-broadband sound-absorbing metamaterials that are suitable for practical applications.