## 2 Materials and Methods
### Preparation of Porous Materials and Performance Evaluation Methods  
A material mirroring the microstructure of the porous sound-absorbing material modeled in Section 2.1 was fabricated. Specifically, non-woven fabrics incorporating particles were produced by mixing pre-mixed polyethylene terephthalate (PET) fibers with a diameter of approximately 7 \(\upmu\)m at 60 wt% and low-melt PET fibers with a diameter of approximately 14 \(\upmu\)m at 40 wt%. Sample Opener OP-400 (Takeuchi Seisakusho Co., Ltd., Osaka, Japan) was used for fiber opening and mixing (Figure 2a). The feed conveyor was set at 0.25 m/min and the cylinder roller at 600 rpm. Each fiber clump, pre-measured to a predetermined ratio, was mixed by hand and placed on the feed conveyor so that it was evenly distributed. The sample opener was used to open and mix the fibers twice. Sample Roller Card SRC-400 (Takeuchi Seisakusho Co., Ltd., Osaka, Japan) was used to card the mixed fibers (Figure 2b). The feed conveyor was set at 0.25 m/min, the cylinder roller at 300 rpm, and the doffer roller at 5 m/min. The fibers mixed by the Sample Opener were placed evenly on the feed conveyor. Carding was performed twice to produce a PET non-woven base sheet with a width of 40 cm and an areal density of approximately 4 g/m\({}^{2}\). A prototype powder feeder (Takeuchi Seisakusho Co., Ltd., Osaka, Japan) for this study was used to spread particles onto a non-woven base sheet (Figure 2c). The powder feeder is equipped with a rotating rubber spatula that shears the particles between the spatula and the walls of the particle stock case and feeds the particles. The bottom of the particle stock case has a hole whose size can be changed, allowing particles of a predetermined size or smaller to be spread at a predetermined rate on a non-woven base sheet with a width of  
Figure 1: Structure of the devised porous material. Figure 2: Non-woven base sheet making equipment: (**a**) Sample Opener; (**b**) Sample Roller Card; (**c**) Prototype Powder Feeder. 40 cm. Aerogel particles (Aerogel P100, 0.1 to 4 mm diameter, approximately 0.15 g/cm\({}^{3}\), Cabot Corporation, Boston, MA, USA) were used. The elasticity of the rubber spatula and the rotation speed were adjusted so that the particle shape and size distribution were within the target range. A PET non-woven base sheet was passed under the powder feeder at a feed rate of 5 m/min to produce a particle spread PET non-woven base sheet. The base sheets were stacked to achieve the targeted bulk density, preheated at 150 \({}^{\circ}\)C for 10 min, and then cold pressed to 10 mm thickness with a flat die at 80 \({}^{\circ}\)C. Thus, a 300 \(\times\) 300 mm PET non-woven fabric test piece and an aerogel particle-mixed non-woven fabric test piece were prepared. A specimen of the same dimensions was also prepared by laminating one side of this non-woven fabric with a high-airflow-resistance skin material (thickness: 0.20 mm) [26], developed jointly with Maeda Corporation (Figure 3). The bulk density, fiber content, and particle content of the test pieces were measured using an electronic balance after the specimens were placed in a crucible and heated in an electric furnace at 600 \({}^{\circ}\)C for 4 h and then sufficiently cooled. Fiber and particle content were derived from the results of the same measurements on test pieces with 100% fiber content and 100% particle content, respectively. Structural analysis of the test pieces was performed using an X-ray CT nano3 DX (Rigaku Corporation, Tokyo, Japan). The normal-incidence sound absorption coefficient was measured using the Impedance Tube Kit Type 4206 (Bruel & Kjaer, Naerum, Denmark; per ISO 10534-2 [27]). The measurement frequency range was 50-1600 Hz. The measurements were calculated in 1/3-octave bands. The inner diameter of the acoustic tube was 100 mm. The diameter of the specimen was adjusted so that the specimen could be set into the acoustic tube without resistance.