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Flip to reveal answersWhat is the Doppler effect (for sound)?
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Question
What is the Doppler effect (for sound)?
Answer
The change in the **frequency (pitch) a listener hears** when the source (or listener) is **moving** — higher on approach, lower on recession.
Question
Source moves towards you — higher or lower pitch?
Answer
**Higher** pitch — the wavefronts bunch up, shortening the wavelength and raising the frequency you hear.
Question
Source moves away from you — higher or lower pitch?
Answer
**Lower** pitch — the wavefronts stretch out, lengthening the wavelength and lowering the frequency you hear.
Question
Does the Doppler effect change the source's own frequency?
Answer
**No** — the source always emits the same f. Only the **observed** frequency f' changes.
Question
What is the given equation for a moving sound source?
Answer
$f' = f\left(\dfrac{v}{v \pm v_{s}}\right)$ — **minus** approaching, **plus** receding. **Given** in the data booklet.
Question
Which sign in v ± v_{s} for an approaching source, and why?
Answer
The **minus** sign — it makes the denominator smaller, so f' is **larger** (higher pitch).
Question
Why does the pitch rise as a source approaches? (wavefronts)
Answer
The source moves forward between emitting each crest, so the **wavefronts ahead bunch together** → shorter wavelength → higher frequency.
Question
What does the heard-frequency-vs-time graph look like as a source passes?
Answer
**High and flat** (approaching) → a **sharp step down** (passing) → **low and flat** (receding). Not a smooth slope.
Question
A car horn passes you — describe the pitch change.
Answer
You hear it **above** its true pitch while it approaches, then a **sudden drop** to **below** its true pitch as it passes and recedes.
Question
Most common Doppler-graph mistake?
Answer
Drawing the heard pitch **sliding down smoothly**. It actually steps **down sharply** at the instant the source passes.
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Doppler effect
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