# Evaluation of a Microporous Acoustic Liner Using Advanced Noise Control Fan Engine +
###### Abstract
These materials have gained significant interest in the field of noise attenuation as they possess channels, cracks, or cavities that allow sound waves to enter the material system. With complex microstructures, the sound energy is dissipated through thermal loss due to the friction of air molecules against the pore walls and viscous loss due to airflow viscosity with the material. Based on these energy consumption principles, porous materials behave like broadband sound absorbers [11; 12]. They are of three types, cellular, fibrous, and granular, based on the microscopic structure [13; 14] (see Figure 4). Among these types of porous sound absorbers, we focused on microstructure-controlled granular porous materials. They are rigid, macro/microscopic particles with dimensions much larger than the interconnected pores, making them suitable for structural applications. The rationale for selecting granular materials is that they allow for the design of porous sound absorbers of engineered acoustic response by controlling the porosity, pore size, and their distribution through the thickness through appropriate selection of particles, and the surface imperfections of the particles improve fluid flow resistivity and tortuosity (the ratio of the actual sound path to the sound path taken). Furthermore, there are acoustic models in the literature that link the microstructural/physical properties (fluid flow properties) of porous materials to acoustic performance. In the frequency domain, all of the acoustic models are based on these fluid flow properties to give a physical basis  
Figure 4: Three types of porous materials: (**a**) cellular; (**b**) fibrous; and (**c**) granular [14]. Figure 3: Conventional DDOF liner [8]. for describing sound propagation through porous media. The commonly used acoustic models, namely Johnson-Champoux-Allard (JCA) [15; 16], Johnson-Champoux-Allard-Lafarge (JCAL) [17], Johnson-Champoux-Allard-Pride-Lafarge (JCAPL) [18], and Pore Size Distribution (PSD) [19], are used to predict the acoustic response, i.e., the absorption coefficient versus frequency and transmission loss versus frequency. We selected natural granular ceramic hollow microbubbles, i.e., Cenospheres, commercially available and supplied by Sphere One--called Extendospheres (commercial name)--which are the mineral waste from coal-burned electric power plants. The rationale for selection is that they are lightweight ceramic particles filled with air, characterized by size, nearly spherical, with a surface roughness that can improve sound energy dissipation by enhancing friction (sound energy converted to heat) and tortuosity. A new particle surface coating technique with a minimal polymer high char binder was developed to mimic the pore properties of the loose granular system to maintain near-similar porosities. First, this concept of material development was demonstrated on solid glass beads (model materials) [20; 21]. Then, processing and fabrication were expanded to lightweight hollow microbubbles, and this new material system is addressed as a microporous hollow bubble (\(\upmu\)PHB) composite material described in detail in references [22; 23]. The developed material is multifunctional, offering thermal, mechanical, and acoustic properties [23]. An extensive experimental and analytical study was conducted on microporous specimens in terms of particle diameter, pore size and their distribution, and specimen thickness [21; 24]. Several different particle size microporous material specimens of 100 mm diameter and 25 and 50 mm thickness were fabricated and impedance tested. Firstly, the particle size must be less than 1 mm to achieve effective acoustic responses, as demonstrated [21; 24]. Then, we concluded that for a given frequency, the absorption is higher for larger-size particles and, alternatively, it is lower for smaller-size particles. Transmission (sound energy) loss directly increased with a decrease in pore size (particle size) and with the increase in specimen thickness.