Low-frequency ultra-broadband absorbers with conical cavity-coupled porous materials

In this paper, a super surface acoustic absorber is developed based on the Fabry-Perot (F-P) resonance cavity for acoustic absorbing super surfaces with deep sub-wavelength thickness. Thanks to its composite structure (TCPM) consisting of a porous material embedded with a tapered cavity super-material (TCS), the absorber achieves good absorption at low frequencies and broad bands without taking into account the parallel coupling of multiple units to each other. A theoretical model for analyzing the absorption characteristics of acoustic absorbers is established, and the absorption mechanism and the effect of parameter changes on the absorption characteristics are studied. The sound absorption coefficient is measured using the impedance tube method, and the experimental results show that under the condition of the deep sub-wavelength thickness (60 mm), a continuous absorption spectrum is realized in the range of 200 ∼ 1600 Hz, and the average absorption coefficient is higher than 0.8. This research offers an effective way to engineer broadband acoustic absorbers with limited space. The developed ultra-surface acoustic materials have the benefits of thin layer thickness, low frequency, and broad bandwidth, which are of great value for use in noise mitigation.