## 3 Results and Discussion
However, a higher frequency peak is recorded at 10 mm of defect, proving that the frequency is sensitive to the presence of defects in the neutral line and especially at 10 mm of defect. Low damping is also observed at 10 mm of defect, which explains why damping reacts differently from peak frequency to defect variation. Figure 10 reveals the average peak frequency of samples as a function of the defect size. The results indicate a notable increase in peak frequency from 1390.055 to 1426.139 Hz when comparing samples without defects to those with a defect dimension of 3 mm squared. This suggests that the acoustic detector effectively detects the presence of defects that are introduced due to better air and structure coupling. In this defect position, local stiffness increases at the hardened edge of the 3 mm defective sample, as shown in Figure 9. This reduces inertia and accelerates response to vibratory forces. From 3 mm to 5 mm of defect, a slight decrease in peak frequency is observed, dropping to 1420.0444 Hz. This is consistent with the fact that the 3 mm samples indicate local hardening, which causes the peak frequency to be higher than it would be with a 5 mm defect, thus breaking the linear relationship between the frequency peak and the defect size. However, from 5 to 10 mm, a slight rise is recorded. This is due to the increase in defect size, particularly in 7 and 10 mm beams that may slightly modify the vibration patterns. In addition, the error bars indicate the dispersion of measurements for each test case. In this respect, these bars overlap between 3 and 10 mm defects, meaning that differences in defect size are not noticeable in this test range. This may be due to the three spatial positions of the defect on the neutral bending axis. As the defect is located on the neutral flexural axis, it interacts less with the propagating wavefronts and has a smaller influence on the modal excitation. Consequently, varying the defect size beyond 3 mm has less of an effect on the peak frequency. The implication of the introduced defect size on the damping response is displayed in Figure 11. A progressive increase in damping is observed as the square void grows from a no-defect state to 5 mm. This elucidates that small 3 and 5 mm defects can create internal sound diffusion and vibratory disorder. Hence, this increases air losses and apparent damping. Conversely damping decreases from 5 mm to 10 mm. This suggests that high defect size (7 and 10 mm) interrupts external wave transmission, and therefore, less energy goes into the air. As the acoustic detector picks up waves radiated by the surface, the acoustic sensor shows low sensitivity to the delamination present in beams of 7 and 10 mm (Figure 7d,e), because the defect size is small compared to the acoustic wavelength, leading to weak scattering. This confirms that damping with an acoustic sensor does not reflect the structural integrity of the 3D-printed part. Based on six taps of three samples for each defect size, Figure 12 displays the average Fourier transform (FFT) curves obtained with MATLAB R2022b software. A fast Fourier transform (FFT) was performed on the vibration signal of the average of 18 taps of all the three samples. To conduct the analysis, a script in MATLAB R2022b was used to visualize the FFT spectra and apply a Lorentzian-type curve fitting. Two distinct peaks can be noticed in Figure 12. The first, identified in red, is the natural frequency, which is a measure of the system's intrinsic resonance. The most elevated frequency between 0 and 1000 Hz is the second peak, named Peak 2, revealing a recurrent vibrational mode in all samples marked with sky blue. In comparison to samples with defects of different sizes, those with no defects in the 0 to 1000 Hz range exhibit a notably higher number of harmonics. These samples therefore offer better wave propagation, and the absence of defects does not disrupt the continuity of the material. For all defect sizes, Peak 2 is shown in Figure 10 at a consistent frequency of 73.27 Hz.