Abstract: Data processing and judgment on the less dense area of concrete and cavity detection in CECS21:2000 standards, is based on that the mean value and standard deviation of the acoustic parameters is in the normal range. But the judgment from the standard should easily lead to omission and miscalculation if the standard deviation is too large or too small. Through the application of engineering examples, it is suggested that the scope of standard deviation should be defined in CECS21:2000 standard if standard deviation is too large or too small and fixed critical value should be used to judge the defect according to the concrete grade level and sound velocity range of the project.