Delay line and immersion transducers utilize a plastic or epoxy delay line, or a water path, between the active element of the transducer and the test piece. The echo from the end of the delay line, or from the front surface of the test piece in immersion setups, is used as the zero reference for depth or distance measurements. (This echo is often referred to as the interface echo.) Otherwise, the calibration procedure is similar to that for single element contact transducers, using two reference samples of known thickness.