## 2 Manufacturing Processes of Musical Instruments Made of Composites and Alternative Materials
### Composites in Musical Instrument Manufacturing  
A composite material is a combination of two or more materials with distinctly different chemical or physical characteristics. Together, they create a new material with enhanced properties and characteristics that neither material can provide on its own [10]. In most composites, one material serves as the matrix, maintaining the shape of the manufacturing and transferring forces to the reinforcement. The main families of materials are ceramic, organic, and metal. Matrices and reinforcements may be derived from each family, and the creation of a composite material can result from any combination of them. Composites may be categorized based on the matrix or the reinforcement [11]. The widely used matrix category in boats, cars, airplanes, bicycles, instruments, and others, is the organic matrix, specifically the polymer matrix. The main categories of composite reinforcements [11], according to their form, are fiber, particle, and structural reinforcement. Reinforcement-oriented characterizations of a composite, such as CFRPs, GFRPs, flex filament, hemp fibers, and others, or matrix-oriented characterizations like fiber-reinforced polyurethane foam (FRUF) and fiber-reinforced epoxy, can generally describe a composite. It is common to define both materials in the description of a composite, especially in the case of a rare combination like carbon-reinforced polyurethane foam (CF/UF). Carbon Fiber (CF) usually has an epoxy resin matrix, and polyester usually has glass fiber reinforcement [11]. Complex composites, such as \"sandwich\", a subcategory of structural reinforced composites [11], can be created from the combination of composites and other materials. In the literature, studies can be found where violins were made from CFRP [12; 13]. Soundboards were crafted from glass and carbon fiber-reinforced polyurethane foam [14], as well as from carbon fiber-reinforced synthetic wood [15]. A drum shell [16] and a cello [17; 18] were manufactured by carbon fiber-reinforced epoxy (CFRE). Various musical instruments made from CFRE, including violin, viola, cello [19], guitar, bass, ukulele [20], snare drum [21], and piano [22], are commercially available. Furthermore, in the manufacture of a traditional stringed Greek bouzouki [23], CFRE on the top and bottom skin, with a core of polyurethane foam in between was considered. The variety of composites is endless and therefore this study gradually focusses on the most used in the musical instrument manufacturing, such as fiber-reinforced polymer matrices. The main form of fiber reinforcements consists of plain fibers without any weave. The most common form of reinforcement in fiber-reinforced composites is fabric, which is created from these fibers. Non-woven fabrics [24] consist of layers of fibers usually sewn with an elastic thin thread in a specific direction, such as unidirectional (UD), biaxial, quadaxial, etc. Woven fabrics [25] can be plain, twill, or more complex in their weave pattern. Polymer matrices are divided into two main categories: thermosets and thermoplastics. Thermoplastics typically undergo numerous state transition cycles (solid to liquid) through warming, with this process having a limited impact on their mechanical properties. Thermoplastics find extensive use in mass-produced items; however, their application as a composite matrix in instrument making is limited [26]. In contrast, thermoset matrices solidify during a curing process and cannot be liquefied again. The curing cycle defines the glass transition temperature, Tg, of a thermoset matrix, representing the temperature at which it remains in a solid state without undergoing plastic deformation while retaining its mechanical properties. Thermoset matrices generally exhibit superior mechanical properties compared to thermoplastics. In the realm of instrument making, many instruments described in the literature are manufactured using thermoset polymers, such as epoxy resin [16; 17; 18].