NP, M3. PrintRite3D is a complete system that uses multiple sensors. I found what is probably the most complete study in using acoustic and optical sensors for in-process monitoring. It's quite long at 228 pages. Nearly every base is covered in theory to experiment and results. The conclusion on page 201 shows the combination of acoustic and optical monitoring having 98% accuracy. This 228 page study independently coincides with Sigma Lab's work. PrintRite3D using acoustic, optical, and thermal sensors for control: