Non-Destructive Testing (NDT) Practice Test

Why is a void considered a better reflector of sound energy than a metallic inclusion?

1. The wave travels slower through the void

2. All always reflects sound equally

3. Greater acoustic impedance mismatch exists

4. Metallic inclusions create less noise

The correct answer is: Greater acoustic impedance mismatch exists

A void is considered a better reflector of sound energy primarily due to the existence of a greater acoustic impedance mismatch between the void and the surrounding material. Acoustic impedance is a measure of how much resistance an acoustic wave encounters when passing through a medium. In the case of a void, which is essentially an air-filled space, the acoustic impedance is significantly lower than that of solid materials like metals or alloys. When a sound wave encounters such a significant difference in acoustic impedance at the boundary between a void and the host material, a considerable portion of the sound wave is reflected back rather than transmitted through. This means that voids can create strong reflection signals that are easily detectable by non-destructive testing methods such as ultrasonic testing. Conversely, metallic inclusions do not provide as stark of a contrast in impedance with the surrounding material compared to a void. While they may reflect some sound energy, the lower difference in acoustic impedance means that less sound energy is reflected back compared to a void. This fundamental difference in acoustic impedance is the key reason for the enhanced reflectivity of sound energy at voids.