Depth calibration depends on how long echoes take to return and how fast sound travels through the medium. When the transducer sends a pulse, the time until the echo returns is a measure of the round-trip distance to a reflector. To turn that time into a physical depth, you need the speed of sound in the phantom medium. A test phantom with reflectors at known spacings gives you exact distances to check against the system’s depth readings. If the speed used to convert time to depth is off, all measured depths will scale incorrectly, even though the timing is accurate. The reflector spacing provides the reference distances, and the propagation speed provides the conversion factor from time to depth. The reflection coefficient and attenuation affect how bright the echoes are, not how deep they appear, so they’re less relevant for depth calibration.