## 2 Manufacturing Processes of Musical Instruments Made of Composites and Alternative Materials
### Three-Dimensional Printing in Musical Instrument Manufacturing  
Three-dimensional printing technology, as an additive manufacturing process, revolutionizes traditional manufacturing processes by building three-dimensional objects layer by layer from digital models [34; 35], allowing for the creation of intricate and complex structures with a high degree of precision. From rapid prototyping to custom manufacturing, 3D printing finds applications across various industries, offering efficiency, cost-effectiveness, and the ability to produce unique designs that may be challenging with conventional pro  
Figure 5: Prototype violins with soundboards from 5 different materials. Reprinted with permission from [12]. Copyright 2020, Acoustical Society of America. cesses. The 3D printing process typically involves three fundamental steps. Firstly, a digital model of the physical object is created, by 3D scanning or conventional measurements and design methods, within a Computer-Aided Design (CAD) [36; 37] system. The developed digital model serves as the blueprint for the 3D printer to follow. The model is sliced to thin layers, using horizontal cross-sectioning with the help of CAD slicing/editing software features. Each of the generated layers represents a cross-section of the final object and dictates the printing pathway layer by layer. The 3D printer deposits or solidifies the chosen material, whether it is plastic, metal, resin, or another substance, to build up the object following the pathway layer by layer, while manual modifications may be required, depending on the complexity of the printed part. Over time, various 3D printing techniques have evolved [35; 38], including Stereolithography (SLA), Selective Laser Sintering (SLS), Fused Deposition Modelling (FDM), Fused Filament Fabrication (FFF), and DLS (Digital Light Synthesis). SLA, the most common resin 3D printing process, constructs objects in a vat of liquid polymer. The liquid resin material is solidified into plastic as a computer-controlled laser beam follows a designated trajectory, heating the polymer. Gradually, the object is constructed layer by layer. SLS builds objects using the powder bed fusion process. In this process, a thin layer of powdered polymer is deposited onto the building platform. A laser or UV light fuses together all particles within the area of the first cross-section. After a layer is created, the platform is lowered, and the process is repeated, producing the object layer by layer. FDM is the most widely used process. It utilizes a thermoplastic filament heated to its melting point by a light source [39; 40; 41]. In DLS, mixed ultraviolet light and oxygen are used to continuously grow 3D parts from a small vat of liquid resin. The material cures during the printing process, resulting in a rigid polyurethane object. The claimed printing time is 25-100 times faster than traditional printers [41]. The materials used in 3D printing are thermoplastic filaments, such as polylactic acid (PLA), polymer powders (usually polyamide), polymer filaments (Polyethylene Terephthalate, PET), photopolymer powders, metal powders and others [42]. A significant challenge in the manufacture of 3D-printed musical instruments lies in the complexity of fabricating such intricate designs. This process demands advanced manufacturing skills and specific tools, typically available only in specialized workshops operated by highly trained craftsmen [43]. In the musical instrument industry, the quality of 3D-printed parts is determined by the mechanical and acoustic standards, set by the manufacturer [5]. While it is feasible to quantitatively measure sound in terms of frequencies, amplitudes, harmonic content, and more, the qualitative assessment of these sounds poses challenges. Subjectivity comes into play when involving human players and listeners, making it challenging to provide an objective evaluation [5; 44].