Ultrasonic inspection involves:

Ultrasonic inspection utilizes high frequency vibrations that are typically above the audible range for humans, often exceeding 20 kHz. This technique employs sound waves to detect imperfections or measure material thickness, which is crucial in various industries, including refrigeration. The high frequency of the vibrations allows for detailed examination of a material's internal structure without causing damage, making it an effective non-destructive testing method.

In contrast to other options, short wavelength radiation and gamma rays pertain to different forms of testing and inspection methods, such as radiographic inspections, which use ionizing radiation. Radiographic penetrameters are also specific to radiography rather than ultrasonic techniques. Thus, the distinction of high frequency vibrations being the correct choice highlights the foundational principle of ultrasonic inspection in its methodology.