在板材厚度为6~16 mm的对接焊缝检测中，超声检测有一定的困难，其原因如下：一是因为薄板焊缝检测区域在近场范围内，超声波声压的变化不像远场区那么有规律，在缺陷定位、定量中容易引起较大的误差。二是薄板焊缝扫查声程较短，仪器与探头连接匹配后杂波多，信噪比差，特别是焊缝中余高引起的反射波，有时超过了判废线，容易引起误判。为此，应采用分段制作DAC曲线的方法进行多次反射法检测。由于采用模拟超声波检测仪分段制作DAC曲线极其不便，而通过采用数字机双闸门法可较好地解决这一技术难题。

In the plate thickness of 6-16 mm butt weld testing, ultrasonic testing has certain difficulties for the following reasons. Firstly, because the sheet weld inspection area was in the near field range, ultrasonic sound pressure changes not as regularly as in the far field zone, resulting in a larger error prone for the defect location and quantity. Secondly, the sound path of sheet metal weld scanning process is shorter, the instrument connected with the probe after the match was subjected with more clutter and poor signal to noise ratio, especially in the weld caused by the reflected waves over the height, sometimes higher than rejection line(RL), easily leading to false conclusion and miscarriage of justice. Therefore, we should make use of sub-DAC curve for detection of multiple reflection method, ultrasonic detector as simulated actual use of sub-DAC curve is extremely inconvenient, and the digital machine double gate method can better solve the technical problems..