You're right! We actually don't use non-conducting mediums to determine the velocity of ultrasonic waves. In fact, we prefer conducting mediums for this purpose. Here's why:

* Ultrasonic waves are mechanical waves: They require a medium to propagate. This medium can be a solid, liquid, or gas.

* Non-conducting mediums: These mediums, like air or certain plastics, have very low densities and compressibilities. This makes it difficult for ultrasonic waves to travel efficiently, and their velocity is typically lower.

* Conducting mediums: Metals, liquids like water, and some solid materials like steel offer excellent properties for propagating ultrasonic waves. They are denser and more compressible, allowing for faster and more efficient wave travel.

Here's why conducting mediums are preferred for determining ultrasonic wave velocity:

* Accurate measurements: The higher speed and better propagation in conductive materials result in more accurate measurements of wave velocity.

* Practical applications: Many practical applications of ultrasound, like medical imaging, non-destructive testing, and material characterization, rely on the interaction of ultrasonic waves with conductive materials.

So, while non-conducting mediums can be used, we usually choose conductive mediums for more reliable and accurate measurements of ultrasonic wave velocity.