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General Considerations
Most ultrasonic tests are conducted in accordance with procedures that limit test parameters within rather narrow boundaries. These procedures are often developed with specific combinations of instruments and transducers.
 Reference Standards, or test blocks, prove the ability of the derived procedures to detect echos from the artificial flaws by using some combination of instrument/transducer.
The Instrument
In most practical circumstances, instrument choice is limited to what is at hand. Ultrasonic instruments are expensive, and, while offered in a range of types produced by a variety of manufacturers, most have many similar characteristics, features, and capabilities. Fortunately, it isn't usually necessary to have many different instruments in order to implement fairly diverse inspection/testing requirements. However, the same cannot be said for ultrasonic transducers.
 The Ultrasonic Transducer is the heart of any ultrasonic test. Test results are highly dependent upon many factors uniquely involving the transducer. Inappropriate transducer selection, or a poorly manufactured or malfunctioning transducer can severely effect test results regardless of instrumentation capability.
Assuming that well-trained competent personnel will perform the required ultrasonic tests using appropriate test instruments, reference standards and high quality, reliable transducer, ultimate test results boil down to a single, most important factor - selection of the most appropriate transducer(s) for the intended test or inspection.
Many factors enter into transducer selection; some are fixed, or measurable, while others may be unknown, uncontrollable, or immeasurable. The more that is known about the physical, mechanical, and geometric properties of the test object, the more precisely can the appropriate transducer(s) be selected. An exhaustive discussion of all such factors in not feasible in this guide, but consideration of the more obvious factors will help. (If you have specific questions, an NDT Instruments technical representative will be happy to assist you with transducer selection.)
It is also helpful t have knowledge of the behavior of soundfields, both within the test object and at it's boundaries. Some of the more important considerations are related to soundbeam frequency, beam spread, near and far-field effects, reflection and refraction phenomena.
MATERIAL FACTORS
In metals, first consideration must be given to the metallurgical structure which may exist. Grain type, size and distribution influence the propagation of the ultrasonic sound beam. Large or directionally oriented grains tend to scatter and otherwise absorb ultrasonic energy to a greater extent than fine, randomly oriented grains. For instance, cast iron will be far more attenuative to ultrasound than aluminum. Some exotic materials may exhibit a directional attenuation effect.
Polymers and elastomers are often highly sound-absorbing or attenuative. Reinforced plastics and other types of multi-phase and composite materials are generally attenuative and produce scattering of the soundfield. Most ceramics are likewise attenuative.
Many times the geometry and surface condition of the test object are the most influencing factors in ultrasonic testing.
Ultrasonic Soundfield Factors
The vibrational frequency (or frequencies) of the transducer element determines the wavelength of the ultrasound within the test material. Combined with the geometry of the element, the frequency also establishes the extent of the near-field or natural focusing point and the amount of beam-spreading in the far-field or the points beyond the focal point.
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