Which of the following is a reference reflector that is not dependent on beam angle?

• A. A flat bottom hole
• B. A vee notch
• C. A side drilled hole which is parallel to the plate surface and perpendicular to the sound path
• D. A disc shaped laminar reflector

Answer: (D)

The idea being tested is that a reference reflector should give a stable echo regardless of the probe’s beam angle. A disc-shaped laminar reflector is designed to do just that. Its laminated, disc-like structure scatters ultrasonic energy in many directions rather than relying on a single, precise reflection path. This makes the back-wall echo relatively insensitive to the incidence angle, so the echo amplitude stays fairly constant as you tilt or rotate the beam. That angle independence is why it’s preferred as a reference standard for calibrating gain and sensitivity across different probe orientations.

In contrast, other reflectors rely on specific geometries that produce strong reflections only at particular angles. A flat-bottom hole gives a strong echo mainly when the beam is near normal to the bottom surface, decreasing as the angle changes. A side-drilled hole aligned with the sound path or a vee notch produces echoes that vary with orientation and angle, making them less reliable as angle-insensitive references.