The term 'velocity ratio' commonly refers to the relationship between which types of waves?

The term 'velocity ratio' is specifically used to describe the relationship between longitudinal waves and shear waves. Longitudinal waves, which consist of compressions and rarefactions, propagate through materials via particle displacement in the direction of wave travel. In contrast, shear waves involve particle movement perpendicular to the direction of wave propagation.

The velocity ratio relates to the speed at which these two types of waves travel through a given medium. In general, longitudinal waves travel faster than shear waves in most materials because they utilize the material's elasticity more effectively. This concept is crucial in understanding wave propagation in NDT, as it helps technicians assess material properties based on the speeds of these waves.

While the other wave combinations presented may have relevant interrelations in different contexts, they do not specifically reflect the established definition of 'velocity ratio' as it pertains to longitudinal and shear waves. Understanding this concept is key when conducting ultrasonic tests and interpreting the results accurately.