A sound wave travels through air from a loudspeaker to a listener. Explain what is meant by the amplitude, frequency, and wavelength of this sound wave, and state how the speed of the wave is related to its frequency and wavelength.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A loudspeaker produces a steady musical note. The sound wave travels through the air at 340 m/s.
Model answer (5 marks)
Amplitude is the maximum displacement of the air particles from their equilibrium position.
Frequency is the number of complete cycles produced per second.
Wavelength is the distance between two successive points that are in phase, i.e. the length of one complete wave.
The speed of the wave is given by v = fλ.
If the frequency increases, the wavelength must decrease (for a constant speed), so frequency and wavelength are inversely proportional.
Frequency is the number of complete cycles produced per second.
Wavelength is the distance between two successive points that are in phase, i.e. the length of one complete wave.
The speed of the wave is given by v = fλ.
If the frequency increases, the wavelength must decrease (for a constant speed), so frequency and wavelength are inversely proportional.
Examiner tips
- Use the exact terminology (maximum displacement, cycles per second, distance between in‑phase points).
- Show the relationship v = fλ and explain the inverse relationship when speed is constant.
Common mistakes
- Confusing amplitude with intensity or sound pressure level.
- Using metres per second for speed but omitting the formula v = fλ.
- Failing to mention that frequency and wavelength are inversely related when speed is constant.
Mark scheme (5 marks)
- Amplitude is the maximum displacement of the air particles from their equilibrium (rest) position
- Frequency is the number of complete waves (cycles) produced per second
- Wavelength is the distance between two successive points that are in phase / the length of one complete wave
- Wave speed equals frequency multiplied by wavelength (v = fλ)
- If frequency increases, wavelength decreases (for a constant wave speed) / frequency and wavelength are inversely proportional at constant speed
Key terms in this question
amplitude · frequency · wavelength
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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