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12.6 The Uses of Ultrasound

GCSE Physics · AQA · Chapter 12: Wave Properties

A free GCSE Physics lesson for AQA (8463) from OBA School: physics only (Higher tier). What ultrasound is, partial reflection at boundaries, medical and industrial imaging, calculating distances from echo times. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • State that ultrasound is sound with a frequency above 20 kHz
  • Explain that ultrasound is partially reflected at a boundary between two media and the echoes can be timed
  • Calculate the depth of a boundary using depth = ½ × v × t
  • Describe medical and industrial uses of ultrasound and explain why an ultrasound scan is safer than an X-ray

Key words

  • Ultrasound: Sound waves with a frequency above the upper limit of human hearing, 20 kHz.
  • Transducer: A device that turns electrical signals into ultrasound pulses and turns returning echoes back into electrical signals.
  • Pulse: A short burst of waves. Sending pulses (not a continuous wave) lets the echo from each pulse be timed.
  • Partial reflection: Part of a wave is reflected at a boundary between two different media and the rest is transmitted.
  • Non-ionising: Does not have enough energy to remove electrons from atoms, so it does not damage cells or DNA the way X-rays can.
  • Flaw: A hidden crack or bubble inside a metal part. It forms a boundary that reflects ultrasound.

What is ultrasound?

Ultrasound is ordinary sound – a longitudinal mechanical wave – but at frequencies too high for humans to hear. Bats and dolphins use it to find their way and their food. Ultrasound for medical scans is made by a transducer that vibrates millions of times a second when an alternating voltage is applied.

Worked example: wavelength of medical ultrasound

A scanner uses 3.0 MHz = 3.0 × 10⁶ Hz. Speed of ultrasound in soft tissue = 1500 m/s.
λ = v ÷ f = 1500 ÷ 3.0 × 10⁶ = 5.0 × 10⁻⁴ m

Answer: λ = 0.50 mm

This very short wavelength is why ultrasound can show small details inside the body.

Partial reflection and timing echoes

When ultrasound reaches a boundary between two different media (e.g. fat and muscle, or steel and the air in a crack), part of the wave is reflected and the rest carries on. So one pulse produces a whole series of echoes, one from each boundary. The deeper the boundary, the longer its echo takes to return.

distance travelled s (m) = speed v (m/s) × time t (s)

depth of boundary (m) = ½ × v × t (the pulse goes there and back)

Worked example: depth of an organ boundary

An echo returns 80 µs after the pulse. Speed in tissue = 1500 m/s. 80 µs = 80 × 10⁻⁶ s
depth = ½ × 1500 × 80 × 10⁻⁶ = 0.060 m

Answer: depth = 6.0 cm

Later echoes are weaker because some energy was reflected at earlier boundaries and some was absorbed by the tissue on the way.

Medical scanning

For a scan, gel is spread on the skin (so that the ultrasound passes into the body instead of reflecting at an air gap) and the transducer is moved across the body. A computer uses the timing and strength of the echoes to build up an image of the boundaries inside. Uses include scans of a baby in the womb, organs such as the kidney or liver, the heart, and damaged muscles and ligaments. Eye surgeons also use it to measure the length of an eyeball before an operation.

  • Ionising?; Ultrasound scan no – safe for a fetus; can be repeated; X-ray image yes – can damage cells, so the dose is kept low
  • Shows; Ultrasound scan boundaries between soft tissues (organs, muscle, fetus); X-ray image mainly bones and teeth (dense tissue absorbs X-rays)
  • How it works; Ultrasound scan reflection of sound pulses at tissue boundaries; X-ray image X-rays absorbed by bone, transmitted by soft tissue

Exam tip: Two reasons ultrasound is used for scans of a baby: it is non-ionising, so it does not harm the baby or the mother, and it is reflected at boundaries between soft tissues, so it can show the baby’s outline and organs.

Industrial imaging: finding hidden flaws

Metal parts such as rail tracks, aircraft wings and welds are checked for cracks before they fail. A transducer on the surface sends ultrasound pulses into the metal. The echoes are shown on an oscilloscope against time: a perfect block gives just a front-surface pulse and a back-surface echo; a crack gives an extra echo part-way between.

Worked example: reading the trace

back surface: depth = ½ × 6000 × 20 × 10⁻⁶ = 0.060 m = 60 mm (the thickness of the block)
crack: depth = ½ × 6000 × 8 × 10⁻⁶ = 0.024 m

Answer: the crack is 24 mm below the surface

Key fact: Shortcut: the crack echo arrives at 8 ÷ 20 = 0.4 of the time of the back-surface echo, so the crack is 0.4 of the way through the block: 0.4 × 60 mm = 24 mm.

Common mistakes

  • Forgetting to halve: v × t is the distance there and back.
  • Forgetting to convert µs to s (× 10⁻⁶) or MHz to Hz (× 10⁶).
  • Saying ultrasound is used because it is “not harmful radiation” without saying why: it is non-ionising.
  • Thinking ultrasound is electromagnetic. It is a (longitudinal) sound wave and needs a medium.

Revision questions

Try these, then check your answers below each one.

Question 1. What is ultrasound? (Choose from: sound with a frequency above 20 kHz, sound with a frequency below 20 Hz, an electromagnetic wave used in scanners, very loud sound.)

Answer: sound with a frequency above 20 kHz

Working: Ultrasound is sound (a longitudinal mechanical wave) with frequency above 20 kHz.

Question 2. A scanner uses 2.0 MHz ultrasound. The speed in soft tissue is 1500 m/s. Calculate the wavelength in millimetres.

Answer: 0.75 mm

Working: 2.0 MHz = 2.0 × 10⁶ Hz; λ = 1500 ÷ 2.0 × 10⁶ = 7.5 × 10⁻⁴ m = 0.75 mm.

Question 3. Why is an ultrasound scan safer than an X-ray for scanning a baby in the womb? (Choose from: ultrasound is non-ionising, ultrasound travels faster than X-rays, ultrasound has a longer wavelength than radio waves, ultrasound is not reflected by tissue.)

Answer: ultrasound is non-ionising

Working: Ultrasound is non-ionising, so it does not damage cells or DNA. X-rays are ionising.

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