A student stands between two parallel walls that are far apart and shouts once. The student hears two separate echoes, one from each wall. Explain why the student hears two separate echoes rather than one, and state what property of sound makes echoes possible.

Cambridge International IGCSE Physics (0625) — 3.4 Sound · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Sound is reflected off each wall. The two walls are at different distances from the student, so the sound wave reaches one wall sooner than the other. Because sound travels at a finite speed, the time taken for the wave to travel to a wall and back is different for each wall. Consequently the two echoes arrive at the student at different times, giving two separate echoes.

The property of sound that makes echoes possible is that sound can be reflected by a surface.

Examiner tips

  • Use the word "reflected" when describing sound at the walls.
  • Mention the different distances of the walls. Use "finite speed" to explain the time difference. Show the link between different travel times and two echoes.

Common mistakes

  • Saying the echoes are caused by sound travelling at different speeds, rather than reflecting. Forgetting to state that the walls are at different distances. Using vague terms like "sound bounces" without specifying reflection.

Mark scheme (4 marks)

  1. Sound is reflected (at each wall / off the surfaces)
  2. The two walls are at different distances from the student
  3. Sound travels at a finite / fixed speed, so it takes a different (longer / shorter) time to reach each wall and return
  4. The two echoes therefore arrive back at the student at different times

Key terms in this question

echo

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