plate stock of uneven and variable thickness is tested for defects by means of ultrasonics whereby the zone adjacent the front surface and up to a depth of the minimum plate thickness is detected conventionally , but under utilization of a novel method the zone of variable thickness adjacent the rear wall is tested by detecting any echo in a gating period that is returned from that zone or from the rear wall , and such an echo is used to set up a supplemental gating period during which either none or the rear wall echo will occur and in the latter case the setting up of the supplemental gating period is interpreted as having resulted from the presence of a defect so that the occurrence of the rear wall echo 9 within the supplemental gating period is registered as an indication of a defect .