## 4 Discussions  
The studies of various sensitive signal sources coming from a 3D printer are aimed at assessing the existing risks from the point of non-invasive information acquisition. The information covers the shape of objects being printed. The existing studies and analyses show that the 3D printer is a source of two signal types that can pose a serious threat to data processing confidentiality [26; 27; 28; 29]. One of them is the acoustic signal [3] generated by stepper motors and printer moving parts (head, platform). Recording them and proper processing allows to identify the direction and sense of the printhead movement and reproduce the objects being printed with great accuracy. This is extremely important for more complex objects. Another type of signal is oscillations (vibrations) measured on the printer housing (Figure 4). The vibrations are generated by the operation of stepper motors installed in the rear part of the printer, while printer oscillations are caused by the movement of printhead and the platform. The recorded oscillation (vibration) signals can support the process of the reconstruction of the objects in which acoustic signals are used. The speed of the printhead movement depends on the type and thickness of the filament used and the shape of the printed object. For the tested 3D printer the printhead movement speeds are as follows:  
* for the directions along the X or Y axis and diagonal line in relation to the X and Y axis (the trihedron): 1.149 \(\left[\frac{cm}{s}\right]\);
* for the directions along the X or Y axis (the tetrahedron): 1.333 \(\left[\frac{cm}{s}\right]\);
* for the arc (the circle): 1.000 \(\left[\frac{cm}{s}\right]\). This is based on the dependencies that define the linear speed [3]: \[v=\frac{s}{t}\left[\frac{cm}{s}\right],\] (1) where: \(v\)--linear speed in \(\left[\frac{cm}{s}\right]\), \(s\)--distance in \(\left[cm\right]\), \(t\)--time in \(\left[s\right]\), and linear speed on the circle with the r radius [3]: \[w=\frac{l}{t}\left[\frac{cm}{s}\right],\] (2) where: \[l=\alpha[rad].r[cm]\] (3)and \(w\)--temporary linear speed in \(\left[\frac{cm}{s}\right]\), \(l\)--arc length in \(\left[cm\right]\), \(t\)--time in \(\left[s\right]\), \(r\)--circle radius in \(\left[cm\right]\), \(a\)--arc angle in \(\left[rad\right]\). The attempts to recreate the printed objects shown in Figure 6 were performed. The time values read (Figure 9) that correspond to printing particular trihedron walls (Figure 7a), tetrahedron walls (Figure 7b) and tube spheres (Figure 7c) are presented in Tables 1-3. The wall lengths and cylinder perimeter values were determined on the basis of these tables (Tables 1-3, Figures 13-15), according to the dependencies (1), (2) and  
The results obtained during tests and analyses allowed us to find that oscillations (vibrations) generated during a 3D printer operation are sensitive signals. They make it possible, similar to acoustic signals, to identify the direction of printhead operation and reproduce the objects being printed. Vibration signals can complement the analyses of acoustic signals. Tables 4-6 include the results of accuracy errors (percentage) when determining the lengths of walls of objects being printed presented in Figure 7. For this purpose, the data obtained during the reproduction of objects based on vibration signals and the following relationship were used [3]:  
\[e=\left|\frac{Z_{w}-Z_{t}}{Z_{w}}\right|\times 100\%, \tag{4}\]  
where: \(e\)--error, \(Z_{w}\)--real length of an object wall being printed, \(Z_{t}\)--reproduced value of an object wall length being printed. Figure 14: The reproduced object in the form of a tube: (**a**) a real shape, (**b**) reconstructed shape. Figure 15: The reproduced object in the form of a tetrahedron: (**a**) a real shape, (**b**) reconstructed shape. Analyzing the data contained in Tables 4-6, it can be seen that the error in recreating the length of the walls of printed objects does not exceed 5%. These are acceptable values.