## 3 Results and Discussion
### Speed of Sound as a Function of Printing Parameters  
Bar charts showing the dependence of longitudinal and shear sounds speeds on nozzle temperature, layer height, and nozzle speed are shown in Figure 7. The error bars on the plots indicate the standard deviation of a single data set. While an increase in nozzle temperature seemed to result in a higher sound speed, the relationship between layer height and sound speed was inversely proportional. Meanwhile, there was no discernible trend in sound speed with increasing nozzle speed.  
Bar charts illustrating the dependence of the calculated Young's and shear moduli on the nozzle temperature, layer height, and nozzle speed are shown in Figure 8. Figure 8 suggests that increasing the nozzle temperature results in better specimen elastic properties, and increasing the layer height resulted in a lower PLA stiffness. This can be attributed to certain reasons. A higher nozzle temperature results in better melting and flow of the PLA filament, which leads to improved interlayer adhesion and reduced porosity, which in turn increases the overall stiffness of the printed specimens. Conversely, an increase in layer height leads to an increase in porosity in the printed structure. A greater layer thickness, as well as lower temperature, can lead to incomplete bonding of the layers, resulting in voids and weakened bonds. In addition, higher layer heights and lower nozzle temperatures lead to larger inhomogeneities and defects such as gaps and delamination, which reduces the stiffness of the material. Conversely, variations in nozzle speed did not present a clear pattern of changes in the elastic moduli.  
Poisson's ratios were calculated using Equation (3) and the results are provided in Table 2. Poisson's ratio is a function of both longitudinal and shear speeds of sound. The interrelationship between these sound speeds defines the value of the Poisson's ratio and, in the printed material, this interrelationship is contingent upon the internal structure of the material such as pores, defects, the quality of the bonding between the layers, etc. Provided that both longitudinal and shear sound speeds follow similar trends, the Poisson's ratio yields comparable dependencies.  
Figure 6: Longitudinal signals collected during repeatability test.  
Figure 7: Longitudinal and shear speeds of sound within the PLA specimens as functions of **(a)** nozzle temperature, **(b)** layer height, and **(c)** nozzle speed.  
Figure 8: Calculated Young’s and shear moduli of PLA specimens as functions of **(a)** nozzle temperature, **(b)** layer height, and **(c)** nozzle speed.  
The experimental measurements of the specimens' density and sound speed, although performed independently of each other, revealed similar trends in response to the varying printing parameters. The observed trends suggest a consistent relationship between the structural characteristics of the printed material and the parameters used in the printing process.  
The longitudinal wave velocity is influenced by the Young's modulus and density (Equation (4)). An increase in the velocity of longitudinal waves correlated with an increase in the Young's modulus of the material. The shear wave velocity depends on the shear modulus and density of the material (Equation (5)). The experimental results confirm this by showing that the shear modulus increased with increasing shear wave velocity and vice versa.