Ultrasonic soundwaves travel at a much lower velocity in water than in most engineering materials of interest. In aluminum and steel alloys, for instance, the longitudinal wave velocity is roughly four times greater than in water; in common plastics and composites, two to three times greater. Because of this velocity difference, the length of the near-field is proportionately greater in water. Beam divergence (beam spread) is also proportionately greater, causing significant energy dispersion over modest water path lengths between the transducer and test object. While, on the one hand, immersion or squirter-type water coupling the soundbeam from transducer to test object is relatively efficient, near-field and beam spread effects must be considered for optimum testing results.
Fortunately, the mere fact that the face of the transducer is in the water permits the soundbeam to be easily focused. Focusing has the dual effect of modifying near-field effects and concentrating the beam energy. Focusing also can enhance soundbeam coupling into curved test object surfaces.
Focusing the ultrasonic soundbeam is accomplished in a manner similar to the way light beams are focused with lenses. An epoxy "lens" bonded to the flat face of a transducer element produces the desired result. Several different lens types have been developed; however, those most commonly used are spherically or cylindrically concave.
In water, with a spherical lens, the soundbeam is focused toward a point. The distance between the lens surface and the focal point depends upon the spherical radius of the lens, as well as the size and frequency of the element. Small radii produce shorter focal lengths. The same is true of cylindrical lenses; the major difference is that the soundbeam is focused toward a line shape. In neither case can the soundbeam be focused precisely to a point or a line. At or near the theoretical focus, the soundbeam maintains shape within a short zone before it again diverges and rapidly spreads out and disperses. Spherically focused beams are column-shaped for a short distance near focus, having a circular cross-section, while cylindrically focused beams have a rectangular cross-section.
At water path distances less than the theoretical focus, if the focused soundbeam enters the test material (having greater sound velocity), the soundbeam converges even more rapidly than in water.
The combined effect of having a convergent beam in water, converging even more so in the test object, concentrates the abailable energy from the transducer into a highly confined region of the area under test. The sensitivity of the resultant focused soundbeam and it's ability to respond to very small discontinuities is therefore much greater than with a divergent soundbeam. However, the volume of test material being instantaneously "illuminated" by a focused beam is substantially smaller than that of a divergent beam. If a large volume of test must be tested, in order to provide complete coverage, the distance between scans (the scan interval) must be very small if focusing is used.
Aside from concentrating the beam energy for test applications requiring small-flaw detection sensitivity, focusing has the added effect of "smoothing" soundwave pressure variations within the near-field. In fact, focal lengths much larger than approximately three-quarters the distance between the theoretical boundary between the near- and far-field have little focusing effect. Longer focal lengths only serve to reduce the effective energy within the soundbeam as compared with an unfocused, flat-faced transducer. There is also a practical minimum focal length that is effective. This distance varies as a combined function of frequency and element size. Click herefor both Maximum and Minimum Practical limits of focal length are presented in the table to the left.
The Maximum Focal Length tabulated is approximately three-quarters of the distance to the terminus of the near-field. This theoretical boundary is called Y+, and is based on the sound pressure produced form a theoretical, single-frequency transducer. Even the most narrowbanded transducers manufactured by NDT Systems radiate soundfields having a range of frequencies. The lower frequency components tend to reduce the effective Y+ distance; hence, the 0.75 Y+ maximum. This effect is even more pronounced with broadband transducers such as NDT Systems' High Resolution (HR) Series. The practical maximum focal length of highly damped transducers tends to be closer to 0.6 Y+.
The Minimum Practical Focal lengths listed in the table can be used to specify transducers useful only for very near-surface applications. Their depth of field is so limited that the soundfield diverges very rapidly beyond the focal point. As a rule of thumb, focal lengths midway between minimum and maximum produce an effective compromise between sensitivity and depth of field.