What type of search units use a plastic wedge or standoff between the transducer and test piece?

The inclusion of a plastic wedge or standoff between the transducer and the test piece is a feature that transitions across several types of ultrasonic testing techniques. In particular, using a wedge allows for better coupling of the sound waves into the test material, which can be critical for achieving accurate readings and improving detection capabilities.

In dual transducer straight beam contact techniques, a wedge is often used to ensure that the ultrasonic beam is directed at the correct angle into the material. This is essential for ensuring that the sound waves penetrate effectively, allowing for thorough inspections of the material's integrity.

Similarly, in angle beam contact methods, wedges are employed to create the necessary angle for the ultrasonic waves to propagate through the material. This is particularly important in applications where the detection of flaws at specific angles is necessary, such as in weld inspections.

For surface wave contact techniques, standoffs can also be utilized to facilitate the propagation of surface waves. Since these waves travel along the surface of the material, having a proper distance between the transducer and the surface can optimize the performance of the test and enhance the detection of surface-breaking flaws.

Therefore, the correct answer encompasses all types of searches presented, as the use of wedges or standoffs is a common practice in enhancing ultrasonic