A student investigates how sound travels through different materials. The student finds that sound travels faster through steel than through air. When a sound wave passes from air into steel, explain what happens to the speed, wavelength and frequency of the sound wave.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sound is a longitudinal wave. The speed of sound in air is approximately 340 m/s. The speed of sound in steel is approximately 5800 m/s.
Model answer (5 marks)
When the wave enters steel the speed increases from 340 m s⁻¹ to about 5800 m s⁻¹. Because the frequency of a wave is fixed by the source, the increased speed must be accommodated by a longer wavelength:
v = fλ → λ = v/f
Thus λ increases. The wave remains longitudinal, so particle oscillations are still parallel to the direction of propagation.
v = fλ → λ = v/f
Thus λ increases. The wave remains longitudinal, so particle oscillations are still parallel to the direction of propagation.
Examiner tips
- State the speed change first, then the wavelength change, finally that frequency is unchanged.
- Use the equation v = fλ to justify the wavelength change.
- Mention that the wave is longitudinal to show understanding of wave type.
Common mistakes
- Claiming the frequency changes; it is set by the source.
- Saying the wavelength decreases; it actually increases with speed.
- Forgetting to mention that the wave remains longitudinal.
Mark scheme (5 marks)
- The speed of the sound wave increases (when passing from air into steel)
- The wavelength of the sound wave increases
- The frequency of the sound wave stays the same / does not change
- Correct reasoning linking speed and wavelength: because frequency is constant and speed increases, wavelength must also increase (use of v = fλ implied or stated)
- Sound is a longitudinal wave so the oscillations / vibrations of particles are parallel to / in the same direction as the direction of travel / energy transfer
Key terms in this question
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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