What basic principle governs the use of sound waves in non-destructive testing?

The principle that governs the use of sound waves in non-destructive testing is centered around how sound waves interact with materials. When sound waves propagate through a material, they travel at different speeds and behave in distinct ways depending on the material's properties and internal conditions.

In non-destructive testing (NDT), sound waves are utilized to detect flaws or inconsistencies by observing how these waves reflect off such changes. When sound waves encounter a flaw—like a crack or void, or variations in material density—they reflect back differently than they would in a flawless section of the material. By analyzing the reflection patterns, testers can infer the presence of flaws without causing any damage to the object being inspected.

This approach is foundational in techniques like ultrasonic testing, where sound waves are used to detect internal defects. The other options do not accurately represent the principles of sound wave interactions relevant to NDT: for example, sound wave speed can vary, but it is the reflection off imperfections that is key to detecting flaws.