A ship uses sonar to detect the seabed. An ultrasound pulse is sent downwards from the ship and an echo is detected 0.6 seconds later. The speed of sound in seawater is 1500 m/s. Explain how sonar works and describe what happens to the ultrasound pulse between being emitted and being detected.

Edexcel A-Level Physics (9PH0) — 4.1 Stress, strain and the Young modulus · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Sonar is used by ships to determine the depth of water beneath them. A pulse of ultrasound is emitted downwards from the hull of the ship.

Model answer (5 marks)

Ultrasound is a sound wave with a frequency higher than the upper limit of human hearing (above 20 kHz). A pulse of this ultrasound is emitted downwards from the ship’s hull and travels through the water towards the seabed. When the pulse reaches the boundary between the water and the seabed it is reflected (bounces back) as an echo. The reflected pulse travels back up to the ship where it is detected by the sonar receiver. By measuring the time taken for the pulse to travel to the seabed and back (0.6 s) and knowing the speed of sound in seawater (1500 m s⁻¹), the depth of the water can be calculated.

Examiner tips

  • Use the term "ultrasound" and give the frequency range to show understanding of the wave type.
  • Explain the path of the pulse (downward, reflection, upward) to cover all stages.
  • Mention the time‑of‑flight and speed of sound to link to depth calculation.
  • Keep the answer concise – 5 marks, so one sentence per point is sufficient.

Common mistakes

  • Confusing the frequency of ultrasound with the frequency of the echo; students may say the echo has the same frequency but should note it is the reflected wave.
  • Failing to mention that the pulse travels both ways (down and back) or omitting the speed of sound; this loses marks for incomplete explanation.
  • Using vague terms like "sound wave" without specifying it is ultrasound or not giving the 20 kHz limit.

Mark scheme (5 marks)

  1. Ultrasound is a sound wave with a frequency higher than the upper limit of human hearing (above 20 000 Hz / 20 kHz)
  2. The ultrasound pulse travels downward through the water towards the seabed
  3. The pulse reflects (bounces back) at the boundary between the water and the seabed
  4. The reflected pulse (echo) is detected back at the ship
  5. The time taken for the pulse to travel to the seabed and back can be used to calculate the depth of the water

Key terms in this question

sonar · ultrasound · echo

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