A student claps two wooden blocks together once near a large flat wall and hears an echo a short time later. Explain why the echo is heard and describe what happens to the frequency, wavelength, and speed of the sound when it reflects off the wall.

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)

The sound wave produced by the clap strikes the wall and is reflected back to the student, producing an echo.

The reflected wave travels back to the source, so it is heard after the original sound.

Because the reflection is elastic, the frequency of the reflected wave remains the same as that of the original wave.

The speed of sound in the air and the wavelength of the wave are also unchanged by the reflection.

Examiner tips

  • Use the word "reflect" or "bounce back" to show understanding of the echo mechanism. Mention that the echo is heard after the original sound. State that frequency, wavelength and speed are unchanged – this is the key point the mark scheme rewards.

Common mistakes

  • Confusing the echo with a new sound source – students sometimes say the wall creates a new sound. Failing to mention that the frequency is unchanged. Using the wrong terminology (e.g. “refracted” instead of “reflected”).

Mark scheme (4 marks)

  1. The sound reflects off the wall (bounces back from the surface / boundary).
  2. The reflected sound travels back to the student, so it is heard after the original sound (as an echo).
  3. The frequency of the sound is unchanged after reflection.
  4. Both the wavelength and the speed of the sound are also unchanged after reflection.

Key terms in this question

echo · frequency · wavelength · speed

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