A series of research has been conducted to analyze the quality of weld seams by non-destructive testing methods. Research on defect analysis using the actual ultrasonic welding phenomenon has not been addressed yet. To ensure a comprehensive quality evaluation, this research estimates the size and location of artificial weld seam defects by the actual ultrasonic welding phenomenon without conventional inspection systems. The welding parameters (power, pressure force, and speed) were carefully selected to ensure optimal bond strength. A superimposed type of seam was produced longitudinally by PVC-coated hybrid textiles in rough to smooth surface contact for two welding groups, using a welding width of 10 mm with an activated cooling air effect. Artificial defects were introduced across the weld seam at five different locations spaced 50 mm apart using Teflon films of 3 and 5 mm width. The actual weld phenomenon of ultrasonic welding process parameters was determined after the recorded machine parameters were converted. The effect of welding process parameters on the seam quality was also analyzed, comparing the weld seam quality between different welding groups and Teflon widths. Based on the discovered graph of the actual welding phenomenon, the location and extent of artificial weld seam defects were estimated. The artificial weld seam defect with a thickness of 0.059 mm at a width of 3.85 mm was estimated at an interval of 50.77 mm using 275 N welding pressure force and 120 W welding power at 2 m min⁻¹ welding speed for 3 mm Teflon width. The results showed that the estimated values closely align with the actual size and position of defects. Overall, this research contributes to the development of a non-destructive testing approach for detecting weld seam defects in ultrasonic welding, emphasizing the importance of these techniques for online inspection and control of weld seam quality.