# Design of a Noise Mitigation System Using Lightweight Graded Micro-Porous Material
## 1 Introduction
### Granular Materials  
Granular materials are rigid, macro/microscopic particles with dimensions much larger than the interconnected pores, making them suitable for structural applications. These materials can be designed for noise mitigation applications by controlling their pore size and their distribution within the material by selecting an appropriate granular size and a proper binder. Further, these material systems can be shaped to various geometric configurations and offer multifunctionality, including structural, thermal, acoustic properties, etc. Acoustic absorption for rubber crumbs [13] in road noise barriers has been studied, but the literature lacks details on sample preparation and transmission loss assessments. [PERSON], [PERSON], and [PERSON] [14] and [PERSON], [PERSON], and [PERSON] [15] studied loose silica aerogel granules; a multilayer configuration showed a better sound attenuation relative to a single layer. Acoustic properties of granular particles like Vermiculite, Perlite, rubber crumbs, and Nitrile foam granulate were tested and reported in reference [16]; here again, the assessments were on loose particles, but the absorption coefficient and transmission loss were not reported. Next, bottom ash (coal combustion residue) and Portland cement binder have been used to demonstrate acoustic performance for highway sound barriers [17; 18]. Lightweight clay aggregates, rubber crumbs, bottom ash, and cement-based noise mitigation materials exist in the literature [19], with a tonal sound absorption response. Similarly, granules of recycled materials used in concrete samples (a--polystyrene used in mc-pol concrete; b--PET used in mc-pet concrete; c--corn cob used in mc-corn concrete) have been assessed for sound absorption (<0.50), with no assessment for transmission loss [20]. However, these high-density materials cannot be used in lightweight applications. Further, aerogel-based materials were assessed [21] for sound absorption on loose particles by filling an impedance tube and were tested using a vertical impedance tube setup; in this work, the transmission loss was reported from experiments wherein the aerogel granules were inserted in sample holders (cylindrical steel cable supports closed at the bottom by a thin, porous py). The loose particle system showed good sound absorption, but the transmission loss varied with the frequency and had a maximum of 16 dB @ 1700 Hz for a 40 mm thick  
Figure 1: Different porous materials for acoustic applications [9]. specimen. The important thing to notice here is the level of complications involved during the testing. The material cannot be used for real applications as it is loose particles. Some other works [22; 23] on loose particles, acoustic absorption using silica-aluminate molecular sieve pellets, and aerogel granules, respectively, are found in the literature. There are limited (with loose particles) or no transmission loss assessments. Also, the few examples of concrete-based shaped granular systems available in the literature are not lightweight. Therefore, based on the literature in terms of material, the selection of lightweight granules and shaping these granules into a solid part using a novel processing technique is a primary challenge. Controlling the microstructure of the porous material to engineer a combination of both sound absorption and transmission loss over a broad frequency range is another challenge. This research addresses the existing gaps in the literature concerning granular porous materials. One novel aspect lies in the selection of lightweight granules with near-spherical shapes and rough surfaces, which can enhance sound energy dissipation through frictional losses, and a complex microstructure to improve tortuosity. Another innovative aspect is the shaping of these granules into a solid porous material without altering the pore properties. Additionally, developing a material that facilitates three-dimensional fluid flow represents a novel contribution.