## 2 Manufacturing Processes of Musical Instruments Made of Composites and Alternative Materials
### Experimental and Numerical Methods for Musical Instrument Vibroacoustic Studies
#### 3.1.3 Vibro-Acoustic Assessments of Musical Instruments with Metamaterials
Their findings indicated that the integration of metamaterials can  
Figure 16: Density distribution of maximum pressure amplitude values of modes on the drum up to 1 kHz for three hammer strike positions (color bar in percentage of maximum density). (**Left**): strike in the ring, (**middle**): strike at ring rim, (**right**): strike outside the ring at the opposite side of the ring. While most maximum values for the strike in the ring are in the ring, few are within the ring when the drum is struck outside the ring. A medium case is found when striking at the ring rim. (Reprinted with permission from [55]. Copyright 2020, Acoustical Society of America. fine-tune the instrument's response without compromising its structural integrity, affirming that employing wooden mechanical metamaterials in classical guitar soundboards is both feasible and advantageous. Figure 17 demonstrates the wooden mechanical metamaterial created by carving specific patterns of perforations into guitar top plates. Figure 18a illustrates the outcomes for the solid top and the two metamaterial variants. Noticeable distinctions are observed in the Helmholtz resonance, occurring at approximately the same frequency due to the minimal volume variation in the air cavity (less than 2%). Additionally, differences are evident in the 400 Hz range, with a more pronounced contrast at 650 Hz, albeit with a lower absolute pressure level, suggesting potential reduced audibility. To emphasize these distinctions, Figure 18b depicts the variations in pressure level. Instruments incorporating metamaterials showcase several advantages over their traditional counterparts. One notable advantage lies in the precise control metamaterials offer over acoustic properties. Unlike traditional materials, metamaterials allow for tailored manipulation of sound frequencies, enabling musicians to achieve desired tonal characteristics with a high degree of precision. Another significant benefit is the ability to finely tune weight and density. Metamaterial structures provide a unique opportunity to create instruments that are both lightweight and structurally robust. This not only enhances the overall playability of the instrument but also contributes to improved portability for musicians. Beyond functionality, the use of metamaterials allows for innovative and customizable designs. Instrument makers can explore modern aesthetics, pushing the boundaries of traditional craftsmanship. Figure 17: **(a)** Diagrams of the soundboards in three different configurations: solid, with longitudinal holes, and with radial holes. With \(p=1\), the major semiaxis of the elliptical holes is 10 mm long, while the minor semiaxis is 2.5 mm long. The figure shows the bracing and the hole patterns, and each surface is colored according to its height with respect to the lower surface of the plate. **(b)** Three-dimensional cut view of the model of the instrument’s body, showing all the individual components. Reproduced under a Creative Commons Attribution 4.0 International License [57]. Copyright 2022, MDPI. A novel method for the experimental evaluation of acoustic metamaterials via the use of Laser Plasma Sound Sources (LPSSs) was proposed in [89]. LPSSs are generated through optoacoustic transduction, achieved by focusing fast (nanosecond) or ultrafast (picosecond to femtosecond) laser pulses with sufficient energy onto a gaseous, liquid, or solid target. In ambient air, the generation of LPSS via laser-induced breakdown (LIB) requires optical intensities surpassing the threshold of approximately 2 \(\times\) 10\({}^{11}\) W/cm\({}^{2}\). The LIB process leads to rapid thermalization in the air due to the interaction of hot free electrons with heavy particles (ions, atoms, and molecules) within the ionization volume. The resulting thermalized air bubble undergoes rapid expansion and contraction, leading to the generation of pressure fluctuations and the emission of an acoustic pulse.