## 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  
[PERSON] et al. [55] presented the characterization and evaluation of an acoustic metamaterial structure consisting of neodymium magnets applied on a mylar drum membrane with a 40 cm diameter. The structure included a ring-shaped area of 10 cm in diameter, isolated by the magnets. The area delimited by the magnets acted as a cloaking mechanism, isolating vibrations within and outside this pre-defined space for certain frequencies. Thus, the waves originating externally or internally to the ring did not traverse the boundary, leading to the formation of an acoustic band gap. Spatially resolved characterization of the metamaterial behavior was achieved both by laser interferometry, which enabled the detection of vibrations on the membrane, and using a microphone array, which enabled the reconstruction of the produced sound field. The drumhead was manually excited by an impulse hammer both inside and outside the separated area. Characteristic results from the evaluation of the pressure amplitude on the membrane via the microphone array for three different excitation positions are shown in Figure 16.  
In [56], the same group demonstrated sound field mapping via the use of a microphone array for the characterization of the vibrational behavior of 3D-printed PLA plates. The plates were modified by attaching various features such as cylinders, rods, and tubes on the surfaces, acting as spectral filters and leading to the formation of band gaps. Moreover, [PERSON] et al. [86; 87] presented a process for the characterization of a mechanical metamaterial structure composed of an array of circular holes (8 mm diameter) arranged in a 6 \(\times\) 4 configuration within a silicone Elite Double 22 fast elastomer matrix. They studied the effect of attaching the metamaterial on the sound box of an acoustic guitar. The characterization process entailed the experimental determination of the metamaterial's frequency response to a vibration excitation. A spectrum analyzer was used to generate sine sweep signals from 300 Hz to 1 kHz, while the signal was amplified by an audio amplifier, which drove a mechanical vibrator serving as the mechanical excitation source. The response of the metamaterial was captured using an accelerometer connected to a signal conditioner. The collected data were directed to the spectrum analyzer that extracted the power spectrum. The process was repeated for different compression levels of the metamaterial, induced by a mechanical press. The influence of the metamaterial on the soundbox of an acoustic guitar was studied by attaching the metamaterial on the sound box using a coupling gel. The experiment was carried out by plucking a string tuned to a specific frequency. The acoustic behavior of the guitar was recorded using a pencil condenser microphone (Samson C02, 12 mm diaphragm, Samson, Hicksville, NY, USA) and an audio interface. The application of the method for the evaluation of the acoustic properties of wooden plates used for the manufacturing of guitar tops was demonstrated in [88]. Holes with different geometrical patterns were drilled in the wooden plates, acting as metamaterial surfaces which can effectively control the intrinsic variability of such wooden tops in terms of their acoustic response. As in acoustic instruments such as the guitar, in this case excitation of the top is airborne and is performed via the use of a loudspeaker, delivering sine sweep signals. Detection was again achieved using an accelerometer. The analysis of the measured signals focused on the detection of the (2, 0) and (0, 2) modes of the surface and was visually performed by observation of the Chladni patterns. [PERSON] et al. [57] investigated the implications of incorporating metamaterials into the soundboards of classical guitars. Through simulations, they assessed how these materials influence modal behavior, sound pressure levels, and the instrument's capacity to withstand string-induced stress.