an on - line , non - destructive method and apparatus monitors a welding process and detects faulty welds by analyzing the average frequency of the airborne radiation emitted from a weld during the welding process . a sequence of measured average frequency values corresponding to sequential blocks of time during the weld are individually compared to a predetermined frequency threshold value . overall weld integrity is discerned by comparing the number of average frequency values exceeding the threshold value to the number of average frequency values not exceeding the threshold value .