A ship uses sonar to map the seabed. Pulses of ultrasound are emitted from beneath the ship, travel through seawater, and are partially reflected back when they reach the boundary between the seawater and the seabed. A receiver detects the returning pulses. Explain how sonar works and why ultrasound, rather than audible sound, is used for this purpose.

OCR A-Level Physics B: Advancing Physics (H557) — 5.2 Circular motion and gravitational fields · Explain · 5 marks · View as Markdown

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

Sonar systems emit pulses of ultrasound from beneath a ship. The pulses travel through seawater and are partially reflected at the boundary between the seawater and the seabed. A receiver next to the emitter detects the reflected pulses, and the time between emission and detection is recorded.

Model answer (5 marks)

Ultrasound pulses are emitted from beneath the ship and travel through seawater. At the interface between seawater and the seabed part of the wave is reflected back to the ship while the rest is transmitted into the seabed. Because the speed of sound in seawater is essentially constant, the time interval between emission and reception of the reflected pulse can be measured. Using the relation (d= frac{1}{2}v,t) (where (v) is the speed of sound in water and (t) the round‑trip time) the depth of the seabed is calculated. The pulses are ultrasonic – their frequency is above 20 kHz, the upper limit of human hearing. Using ultrasound rather than audible sound prevents interference with the crew and with marine life that can hear lower frequencies, and the higher frequency allows the beam to be more tightly focused, giving a clearer image of the seabed.

Examiner tips

  • Show the time–depth relationship (d= frac12vt) to earn the depth part. Mention the constant speed of sound in water. Include the 20 kHz cut‑off to justify ultrasound. Explain the benefits of ultrasound for crew and marine life and for beam focusing. Keep each point concise and use exam terminology.

Common mistakes

  • Forgetting to divide the travel time by two when calculating depth. Using audible sound frequencies instead of stating the 20 kHz threshold. Not mentioning the constant speed of sound in water or the benefit of a narrow beam.

Mark scheme (5 marks)

  1. Ultrasound pulses are partially reflected at the boundary between two media (seawater and seabed)
  2. The remainder of the waves are transmitted (continue through) the seabed
  3. The speed of the ultrasound waves is constant (in a given medium), so the time between emission and detection of the reflected pulse is used to determine the depth of the seabed
  4. Ultrasound has a frequency above 20 kHz, which is above the upper limit of human hearing
  5. Using ultrasound rather than audible sound means the sonar signal does not interfere with or cause disturbance to crew or marine life that can detect audible frequencies / ultrasound can be directed as a narrow pulse more effectively for imaging

Key terms in this question

ultrasound · boundary · partially reflected · sonar

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