Optimal sound-absorbing structures

Abstract

Causal nature of the acoustic response, for any materials or structures, dictates an inequality that relates the absorption spectrum of the sample to its thickness. We present a general recipe for constructing sound-absorbing structures that can attain near-equality for the causal relation with very high absorption performance; such structures are denoted "optimal." Our strategy involves using carefully designed acoustic metamaterials as backing to a thin layer of conventional sound absorbing material, e.g., acoustic sponge. By using this design approach, we have realized a 12 cm-thick structure that exhibits broadband, near-perfect flat absorption spectrum starting at around 400 Hz. From the causal relation, the calculated minimum sample thickness is 11.5 cm for the observed absorption spectrum. We present the theory that underlies such absorption performance, involving the evanescent waves and their interaction with a dissipative medium, and show the excellent agreement with the experiment.

Optimal sound absorbing structures

“…In both cases we find our optimal structures to have better absorption within the target frequency range (1,2). Such advantage is the result of the design strategy.…”

Breaking the barriers: advances in acoustic functional materials

“…In particular, the resonance-induced narrow working bandwidth and inherent losses are limiting factors for the applications of acoustic metamaterials. One possible solution could be the designed integration of resonances as in the realization of the broadband acoustic absorber [194]. Another possible solution is to utilize tunable active metamaterials [211,212].…”

“…Many attempts have been made along this direction [195][196][197]; however, the lack of a central integration principle results in only limited success. Recently, work on the causally optimal broadband absorber (COBA) has solved the problem by integrating the causal constraint, equation (7), into the absorbers' design strategy [194]. A causally optimal absorber is the one that can take the equals sign in equation (7).…”

“…[163], the full width at half maximum of the absorption peak is only about 1/127 of the central frequency. This has recently been understood through the causality constraint in the sound absorption process: measuring the sound REVIEW pressure level dropping during scatterings on an absorber by decibels (dB), its integral over all the wavelengths has an upper limit that is proportional to the absorber's thickness [173,194]:…”

Sound absorption by acoustic microlattice with optimized pore configuration

“…This makes the optimization on intrinsic structures of the SAMs targeting maximum sound absorptivity a challenging task, and also makes previous efforts to prefer to use flow resistance as the intermediate variable, instead of the material morphology, in SAM optimization. 8 Recently, metamaterials or metasurfaces [9][10][11][12] based SAMs providing manageable and adaptable performance are attracting growing interest; some may provide dark acoustic effect at low frequency 11 or ultra-broadband acoustic absorption. 12 These SAMs possess well-defined and organized microstructures.…”