## 7 Results and Discussion  
Three carbon fiber bouzoukis were manufactured, and their vibrational behavior was compared to W02 via ESPI measurements. Figure 4 shows the graphs of displacement vs. the frequency of W02 compared to the C01, C02, and C03 bouzuki for 90 dB SPL excitation. The ESPI experimental results presented were also validated by impulse response measurements. Figure 5 shows the FRF results of the impulse response for the C01 bouzuki in relation to a representative ESPI experimental result. The identification of the mode (1,2) at 450 Hz (red spot) and the corresponding modal shape provided by the ESPI at the same frequency is also demonstrated. The vibration analysis results presented in Figure 4 indicate that the carbon fiber bouzuki C01 behaved acoustodynamically, similar to W02, for many frequencies, while the vibration behavior of the bouzuki C02 and C03 deviated from W02. The vibrational behavior of the C01 and W02 musical instruments was similar for frequencies up to 1000 Hz, and a good agreement was observed for frequencies higher than 1000 Hz. Therefore, C01 was further studied with reference to the W02 bouzuki in terms of the emitted sound and playability via psychoacoustic tests. Figure 3: Front (**a**) and back (**b**) side view of the bouzuki discretised soundboard. Figure 4: Displacement vs. frequency of W02 compared to C01 (**a**), C02 (**b**), and C03 (**c**) carbon fiber bouzouki for 90 dB SPL measured by ESPI, including the fundamental string frequencies for 3- and 4-string bouzouki. Figure 6 shows the results of the psychoacoustic test performed by the five soloists for the musical instruments W02 and C01. The average score results indicate that the wooden bouzouki excels in full sound, warm sound, bass, balanced behavior, ease of play, and response, while the carbon bouzouki excels in bright sound, treble, loudness, and attack. It should also be mentioned that in the second part of the test, the 60 participants gave an average score of 3.82 for the classic bouzouki and 3.65 for the carbon fiber bouzouki and for all the parameters that characterized the sound behavior of the instruments. The psychoacoustic tests for the reference traditional bouzouki W02 demonstrated that the sound and playability of the wooden instrument were rated slightly better compared to the C01-qualified carbon fiber. Figure 5: FRF experimental results (**left**) and an ESPI representative result (**right**) for the mode (1,2), at 450 Hz of the carbon fiber bouzouki C01. Figure 6: The average score (5 is the max. value) of W02 and C01, received from the five soloists for the parameters that characterize the sound and playability of the bouzouki. The FEM model of C01 was developed as described in Section 5. The modal FEM simulation results of the resonant frequencies of six vibrational modes compared to the experimental ESPI results are presented in Table 2. The simulation results agreed well with the experimental results, while the maximum deviation was indicated to be less than 6%. This difference could be mainly attributed to the uncertainty of the material properties adopted in the simulations. Figure 7 shows four representative modal shapes obtained by the FEM simulation results in comparison with the ESPI experimental results. Aiming to optimize the manufacturing process of the bouzouki by minimizing the time and the production cost, the replacement of the wooden bridge and bars with carbon fiber was studied by FEM. The results of the simulation for the same model and bridge and bars made from Twill carbon fiber (FEM1) are presented in Table 3 with reference to the ESPI W02 traditional bouzouki experimental results. Furthermore, a different number of layers for the soundboard, along with different carbon fiber directions, were also tested.