The purpose of a couplant is to match impedances between the transducer and test object

• A. Increase transmitted frequency
• B. Protect the transducer from immersion
• C. Match impedances between transducer and test object
• D. Improve receiver sensitivity

Answer: (C)

Impedance matching at the boundary between the transducer and the test object is what the couplant provides. It fills the gap with a medium that has an acoustic impedance closer to both surfaces, which reduces reflections that would occur if air were present. When there’s a large mismatch (as with air), most of the energy bounces back and little enters the material, degrading the signal. By choosing a couplant that lies between the transducer and the test object in impedance, the ultrasonic wave energy transfers more efficiently into the test object, producing a stronger, clearer signal on reception. The transducer’s frequency is set by the piezoelectric element, not by the couplant, and while the couplant can affect signal quality, its primary job is enabling efficient energy transfer through impedance matching.