In PAUT, what does the term "Doppler effect" refer to?

In Phased Array Ultrasonic Testing (PAUT), the term "Doppler effect" specifically refers to a change in frequency or wavelength of waves in relation to an observer moving relative to the source of the waves. This phenomenon occurs when the source of sound waves is in motion, causing frequencies to shift higher as the source approaches the observer and lower as it moves away.

Understanding the Doppler effect is crucial in various applications within ultrasonic testing and medical imaging, as it can provide significant information about the motion of defects or changes in material properties. In the context of PAUT, recognizing frequency shifts can help in assessing the characteristics of flaws, which is essential for accurate evaluation and interpretation of inspection results.

The other options do not accurately depict the nature of the Doppler effect. For example, absorption of ultrasonic waves relates to energy loss in materials and is not connected to frequency changes due to motion. Harmonics generation pertains to the creation of new frequency components but does not involve the relative motion essential for the Doppler effect. Lastly, while sound wave reflection at boundaries is a fundamental principle in ultrasound, it focuses on wave behavior at interfaces rather than the frequency shift associated with motion.