UT makes use of devices called transducers which can convert input electrical energy to sound and vice versa. Some simple commercially-available transducers are pictured below. These each have a single piezoelectric element and come in different styles. The ones pictured below are specially designed for immersion in water. In so-called “immersion ultrasonics” a water layer between the transducer and the component being inspected serves as a good medium for conducting sound into the component. Also by varying the water path for focused transducers, one can control how deeply the sound is focused within the component.
Other varieties of transducers are designed to be used “in contact”, that is with the transducer directly pressed against the component, usually with a thin layer of coupling fluid. Inspections using contact transducers tend to be less reproducible since the pressure with which the transducer is pressed against the surface affects the strength of the measured signals. For industrial immersion and contact inspections, the frequency of the sound waves is generally between 0.5 megahertz (MHz) and 25 MHz, although higher and lower frequencies are sometimes used in special applications.
Some transducers have either a curved piezoelectric element or a curved lens to produce a focused sound beam. The focal length in water is fixed, being determined by the transducer construction.
Another type of transducer is a “phased array”. This contains many small piezoelectric elements for converting electrical energy into sound. By “phasing” the various elements (i.e., firing them at slightly different times) one can form sound beams focused at different depths. Thus the focal characteristics of a phased-array transducer can be varied (within limits).
Both the water layer and the metal component are 50 mm (about 2 inches) thick. Here the transducer is operated in continuous mode rather than in pulsed mode. In the top view the elements are phased to focus sound near the center of the component. In the bottom view, the top eight elements are turned off, resulting in a decrease of sound intensity at the focus, and a skewing of the sound pattern.