Doppler effect for sound waves
Cambridge A-Level Physics (9702) · Unit 7: Waves · 7 flashcards
Doppler effect for sound waves is topic 7.3 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 7 — Waves , alongside Progressive waves, Transverse and longitudinal waves and Electromagnetic spectrum. In one line: When a source of sound waves moves relative to a stationary observer, the observed frequency of the sound is different from the frequency emitted by the source.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 7 flashcards — 2 definitions, 3 key concepts and 2 calculations — covering the precise wording mark schemes reward. Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Doppler effect for sound waves in the context of a moving source and a stationary observer
When a source of sound waves moves relative to a stationary observer, the observed frequency of the sound is different from the frequency emitted by the source.
What the Cambridge 9702 syllabus says
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Understand that a sound source moving relative to a stationary observer gives an observed frequency that differs from the frequency emitted. The case of a moving observer and stationary source is not required.
- Use fo = fs v / (v ± vs) to work out the frequency heard by a stationary observer when a sound source is moving relative to them.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
State the Doppler effect for sound waves in the context of a moving source and a stationary observer.
When a source of sound waves moves relative to a stationary observer, the observed frequency of the sound is different from the frequency emitted by the source.
A sound source is moving towards a stationary observer. Will the observed frequency be higher or lower than the source frequency?
The observed frequency will be higher than the source frequency. As the source approaches, the sound waves are compressed, leading to a shorter wavelength and thus a higher frequency.
Write the formula for calculating the observed frequency (f₀) when a sound source moves relative to a stationary observer.
f₀ = fₛv / (v ± vₛ), where fₛ is the source frequency, v is the speed of sound, and vₛ is the speed of the source. The '+' sign is used when the source is moving away and the '-' sign is used when the source is moving towards.
A car horn emits a sound at 400 Hz. The car is moving towards you (a stationary observer) at 20 m/s. The speed of sound is 340 m/s. Calculate the frequency you observe.
Using f₀ = fₛv / (v - vₛ): f₀ = (400 Hz * 340 m/s) / (340 m/s - 20 m/s) = 425 Hz (approximately).
A train is moving away from a stationary observer at 30 m/s and its whistle emits a sound at 500 Hz. If the speed of sound is 340 m/s, what frequency will the observer hear?
Using f₀ = fₛv / (v + vₛ): f₀ = (500 Hz * 340 m/s) / (340 m/s + 30 m/s) = 459.5 Hz (approximately).
Explain why the observed frequency changes when the sound source is moving.
As the sound source moves, it 'catches up' with the sound waves it emits in the direction of its motion, effectively compressing the wavelength. In the opposite direction, the source moves away from previously emitted waves, stretching the wavelength. Frequency is inversely proportional to wavelength.
What happens to the observed wavelength as a sound source moves towards a stationary observer?
The observed wavelength decreases (gets shorter). This is because the source is 'catching up' with the sound waves it is emitting, effectively compressing them.
Review the material
Read full revision notes on Doppler effect for sound waves — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 7 — Waves
Doppler effect for sound waves sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Doppler effect for sound waves deck
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