Stepwise gradient acoustic liners for engine noise reduction applications

Abstract

Aircraft noise has been a persistent issue to the communities around the globe. Conventional acoustic liners used to attenuate aircraft engine noise are only effective over a narrow frequency range and considered poor broadband sound absorbers. Metal foams, on the other hand, have a greater potential to replace these conventional liners with their multifunctional properties. Our initial study with metal foams with increasing relative density, compression ratios, and decreasing pore sizes showed that the acoustic properties—the sound absorption coefficient and sound transmission loss—improved over a broad frequency range. Further, we studied stepwise relative density gradient aluminum foams with increasing and decreasing order of compression ratios along the thickness. Our results showed that these configurations have a greater potential to offer lightweight alternatives to uniform foams with better acoustic performance, and with the ability to be tailored acoustic performance over the frequency range of interest. Although metal foams offer multifunctional properties, maintaining consistency in the fabrication process with repeatability is still a challenge. The factors of randomness and irregularity cannot be fully ignored yet as the acoustic properties are affected significantly with changes in microgeometry. Alongside, the advancements in additive manufacturing technologies have pushed the boundaries on realizing the true potential of manufacturing complex micro- and nano-geometries for various applications. Here, we focus on Stereolithography technique, understand the material behavior of the clear resin, and use hybrid modeling methods to extract the sound absorption properties. Our results show a significant difference in its acoustic behavior when compared to the experiments. We also realize that not all the acoustic energy dissipation is being captured; hence, we propose using a correction factor to be able to understand and predict the true nature of additively manufactured porous absorbers with uniform and stepwise relative density gradients along the thickness.