Abstract
Materials with hierarchical internal structures demonstrate prominent properties and versatile functionalities in various fields. One exceptional case is in noise suppression that, striking enhancement of low frequency acoustic absorption is achievable by virtue of hierarchical porosity. However, the mechanism underlying such enhancement yet requires deeper investigation, and attaining desirable acoustic performance by tailoring porous morphology remains challenging. In this work, we present a material realized by template-constraint pyrolysis of degreased cotton, to controllably tune the hierarchical porosity to improve acoustic absorptivity over selective frequency ranges. Through experiment measurement and numerical modeling, three key acoustic air flowing modes are characterized with each respectively corresponding to low, middle and high frequency absorption. Based on these findings, ultra-low and ultra-broad acoustic absorption is realized by forming in the carbonized material with conical macro-pores, instead of simply straight through pores without cross-section variation.
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