Concrete is the most essential material in the construction industry. Assessing its structural integrity is crucial for ensuring safety and durability. Non-destructive ultrasonic testing is commonly employed to detect defects in concrete. However, the presence of multiple defects can significantly affect the accuracy of these methods. Hence, the aim of this study is to evaluate the effects of multiple defects in terms of their arrangements, types, and sizes, on ultrasonic signal arrival time in concrete models, by using COMSOL Multiphysics 6.0. Two groups of concrete models were evaluated: Group A consisting of close, non-linearly arranged defects, and Group B with linearly arranged defects distanced 100 mm apart. Each group comprised six models with varying combinations of air hole and rust defects. The ultrasonic signal arrival times were measured using the through-transmission method. The results indicate that distanced, linear arrangement defects cause longer delays compared to close, non-linear configurations. The findings provide insights for enhancing non-destructive ultrasonic testing techniques for detecting and assessing structural defects in concrete.