## 2 Manufacturing Processes of Musical Instruments Made of Composites and Alternative Materials
### Experimental and Numerical Methods for Musical Instrument Vibroacoustic Studies
#### 3.1.2 Vibro-Acoustic Assessments of 3D-printed Musical Instruments  
The elastic properties of a 3D-printed component or structure play a crucial role in determining its vibro-acoustic behavior [42]. These properties depend not only on the material itself but also on the structure's internal filling. Other material properties such as density, internal damping, and homogeneity can significantly impact the vibro-acoustic behavior. The geometric shape, size, and overall design of the structure can influence how vibrations are transmitted. The level of precision in the 3D printing process, including layer resolution and material deposition consistency, can affect the structural integrity and vibrational characteristics. Parameters such as printing speed, layer thickness, and infill density during the 3D printing process influence the internal structure and, consequently, the vibro-acoustic properties. [PERSON] et al. [80] proposed a technique for estimating the elastic properties of 3D-printed materials, via fused-filament-fabrication (FFF), using modal-impact-sound analysis. They performed FEM-BEM simulations that considered the material anisotropy and estimated the material elastic properties by minimizing the residuals between the simulated and recorded modal impact sound features. The impact sounds were recorded by a microphone. Figure 11 demonstrates the modal sound synthesis process. [PERSON] et al. [36] performed impulse measurements and FEM simulations to study the vibro-acoustic performance of simplified string instrument bodies/resonators. Figure 10: CFRE drum shell amplitude vs. frequency obtained by modal FEM and experimental analysis (lsr stands for the length of the structural reinforcement and N the number of layers). Reproduced under a Creative Commons Attribution 4.0 International License [16]. Copyright 2019, MDPI. Figure 9: Vibrational modes (m, 0) of drum shell obtained from FEM modal simulations. Reproduced under a Creative Commons Attribution 4.0 International License [16]. Copyright 2019, MDPI. Numerous researchers have explored and developed 3D-printed musical instruments based on the acoustic properties of thermoset materials and their internal structures. In a study conducted by [PERSON] and [PERSON] [49], the acoustical behavior of 15 PLA (filament) claves was investigated, with variations in dimensions and designs, including filled, pipe, and void claves. Claves, characterized by their simple cylindrical geometry and suitability for hand-played percussion instruments, are convenient for testing. The researchers adopted a two-pronged approach, comparing experimental recording results with both mathematical analysis predictions and FEM simulations. Their findings indicated that frequencies of PLA claves could be accurately predicted in cases with a filled design but not in cases with a pipe design. Although PLA claves approximated the typical size of traditional claves, they could not easily replicate the acoustic characteristics, particularly the pitch, of classical wooden claves. [PERSON] et al. [81] designed and developed a hand-cranked music box base made from PLA and manufactured via 3D printing. Experimental recordings demonstrated that the sound spectrum closely resembled the original at frequencies within the range where human hearing is most sensitive (500-4000 Hz). Furthermore, FEM results demonstrated that the loudest and best sound quality can be achieved using a 60\({}^{\circ}\) angle slope for the music box base structure. [PERSON] et al. [82] designed and printed a keyboard instrument, specifically a 3D-printed grand piano action and a self-tuning controller, with the intention of advancing the modern piano. However, further refinement was needed for its optimal functionality. [PERSON] et al. [42] designed and developed a high-quality violin with the capability of being 3D printed and assembled at a low cost. Throughout the project, multiple iterations  
Figure 11: Modal sound synthesis process.