Engineers use ultrasound waves to inspect metal pipelines for internal cracks without cutting the pipes open. Explain how ultrasound waves can be used to detect a crack inside a metal pipeline, and why ultrasound rather than audible sound is chosen for this purpose.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Industrial pipelines carry high-pressure gases and must be regularly inspected for internal cracks. Engineers use a non-invasive technique that involves directing waves into the metal from outside the pipe and analysing the reflected signals.
Model answer (5 marks)
Ultrasound waves are emitted from a transducer placed on the outside of the pipe.
When the waves reach a discontinuity such as a crack, part of the wave is reflected back towards the transducer.
A receiver detects the reflected wave and the time interval between emission and detection is measured.
Because the crack is closer to the transducer than the far wall of the pipe, the reflected wave from the crack arrives earlier (or after a shorter travel time) than the reflection from the pipe wall, allowing the crack’s position to be calculated.
Ultrasound is used instead of audible sound because its frequency is above 20 kHz, giving a much shorter wavelength. The shorter wavelength provides higher spatial resolution, enabling the detection of smaller cracks that would be invisible to audible‑frequency waves.
When the waves reach a discontinuity such as a crack, part of the wave is reflected back towards the transducer.
A receiver detects the reflected wave and the time interval between emission and detection is measured.
Because the crack is closer to the transducer than the far wall of the pipe, the reflected wave from the crack arrives earlier (or after a shorter travel time) than the reflection from the pipe wall, allowing the crack’s position to be calculated.
Ultrasound is used instead of audible sound because its frequency is above 20 kHz, giving a much shorter wavelength. The shorter wavelength provides higher spatial resolution, enabling the detection of smaller cracks that would be invisible to audible‑frequency waves.
Examiner tips
- Use the word "ultrasound" and specify the transducer emits waves from outside the pipe. Show the reflection and time‑of‑flight idea. Explain that a crack gives an earlier return. Mention the 20 kHz cut‑off and shorter wavelength for better resolution.
Common mistakes
- Confusing audible sound with ultrasound; forgetting the 20 kHz threshold. Failing to explain that the crack gives an earlier reflected signal. Not mentioning the shorter wavelength and higher resolution of ultrasound.
Mark scheme (5 marks)
- Ultrasound waves are emitted into the metal pipeline from outside
- When the waves reach a boundary (e.g. a crack), they are partially reflected back
- A receiver detects the reflected waves and the time between emission and detection is measured
- A crack reflects waves earlier / after a shorter time than the far wall of the pipe, so its position can be located
- Ultrasound has a frequency above 20 kHz (above the upper limit of human hearing) giving a shorter wavelength, which allows smaller cracks to be detected / gives better resolution than audible sound
Key terms in this question
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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