# Sonic Crystal Noise Barrier with Resonant Cavities for Train Brake Noise Mitigation
## 2 Materials and Methods
### Experimental Set Up  
In order to validate the resonant SCNBs experimentally, a 3D-printed prototype on a 1:2 scale was produced from polylactic acid (PLA) material. Three-dimensional printing is an advanced manufacturing technique that involves building three-dimensional objects by depositing a material layer by layer. The 3D model was created using computer-aided design (CAD) software, specifically Rhinoceros 3D (v.7.19.22165.13002) [46], to facilitate the design processes. The scaled prototype was produced using fused deposition modelling (FDM) 3D printing technology, as depicted in Figure 2. In particular, Figure 2a displays a frontal view of the SCNBs with the local resonator at 0\({}^{\circ}\), while Figure 2b presents a top view of the SCNBs in the two configurations, 0\({}^{\circ}\) and 90\({}^{\circ}\), respectively, on the left and right. Due to the size of the 3D printer, the SCNB was printed in two identical modules. One of them is shown in Figure 2a and the entire SCNB is shown in Figure 2b.  
The Ultimaker S5 (Ultimaker, Utrecht, The Netherlands) was used as the 3D printer due to its large printing area and high printing quality. Ultimaker PLA filament served as the printing material, processed at 220 \({}^{\circ}\)C with a 0.4 mm-diameter nozzle and a heated bed at 60 \({}^{\circ}\)C. The layer height, indicative of printing resolution, was set at approximately 100 microns. The slicing and toolpath instructions for the Ultimaker S5 were generated using CURA (version 4.13.1), an open-source slicing software selected for its compatibility with the Ultimaker S5 3D printer. CURA effectively transforms the 3D model into data suitable for execution on the Ultimaker S5 3D printer. The data are transferred in G-code format to the 3D printer, which then receives all the control information to three-dimensionally produce the prototype.  
To assess the noise insulation produced by the SCNBs and to verify the importance of the orientation of the resonant cavity with respect the noise source, acoustic experiments were carried out in the anechoic chamber of the Higher Polytechnic School of Gandia. In addition, a conventional flat barrier of the same size is tested to compare the results and observe the upper and border edges diffraction limit. The dimensions of the scaled barriers are 21 cm in height, 11.5 cm in depth and 36 cm in width. A Genelec 8030A active loudspeaker is used as the sound source positioned 30 cm from the centre of the barrier, being the same distance as the 2D numerical model. Two condenser microphones, Behringer ECM8000, are used to measure the acoustic pressure at 6 cm and 10 cm of distance from the barrier. The distances of 6 cm and 10 cm were chosen to ensure placement within the \"shadow zone\" of the barrier according to its dimensions. Two different measurement heights at 10 cm and 12 cm were chosen to avoid possible resonances due to same wave path length. Before using them, they were calibrated with the Bruel & Kjaer Sound calibrator type 4231 Class 1 (Bruel & Kjaer, Narum, Denmark), a device designed for the accurate in-field acoustic calibration of microphones and other sound measurement equipment.  
They are located in the centre at a distance of 8 cm from the central edge and at a distance of 10 cm on the right and left sides of the barrier as shown in Figure 3. At these positions, the CB, the 0\({}^{\circ}\) SCNB, the 90\({}^{\circ}\) SCNB and the case without the barrier are measured to obtain the IL results.  
The measurements were carried out with an M-Audio M-Track MKII soundcard and emitting sine sweeps signals from 2 to 9 kHz. In all barriers, three sine sweeps measurements were averaged. This type of signal facilitates the assessment of the impulse response for a time-invariant linear system by cross-correlating the recorded signal from the microphone.