Distance Calculation: The distance to a boundary is calculated using the speed of sound in the medium () and the time taken for the echo to return ().
The Round-Trip Factor: Since the pulse travels to the boundary and back, the total distance traveled is . Therefore, the depth () of the boundary is given by:
Constant Speed Assumption: Accurate measurement relies on the assumption that the speed of sound is constant within the specific medium being tested (e.g., soft tissue or steel).
Non-Destructive Testing (NDT): Ultrasound is used to inspect the integrity of materials, such as metal blocks or rail tracks, without damaging them.
Flaw Detection: Internal cracks or voids create a new boundary within the material. This results in an echo returning to the transducer earlier than the echo from the back wall of the object.
Oscilloscope Analysis: In industrial settings, the results are often viewed as pulses on an oscilloscope trace. The position of the pulse on the time axis corresponds to the depth of the feature causing the reflection.
Medical vs. Industrial: Medical ultrasound typically constructs visual images from complex reflection patterns, whereas industrial ultrasound often analyzes specific pulse timings to locate discrete flaws.
Transmission vs. Reflection: While reflection is used for imaging, transmission is necessary for the wave to penetrate deeper into the object. If a material absorbs or reflects 100% of the wave at the surface, internal imaging is impossible.
The 'Halving' Trap: The most common error in calculation questions is forgetting that the time given is usually for the round trip (source object source). You must either halve the time or halve the calculated total distance to find the depth.
Unit Consistency: Ensure units are consistent. Speed is often in , but time might be in microseconds () or distance in centimeters (). Always convert to standard SI units before calculating.
Trace Interpretation: On an oscilloscope diagram, the horizontal axis represents time (and thus distance). A pulse appearing between the transmission pulse and the back-wall pulse indicates an internal defect.