Doppler effect for sound

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The change in the **frequency (pitch) a listener hears** when the source (or listener) is **moving** — higher on approach, lower on recession.

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**Higher** pitch — the wavefronts bunch up, shortening the wavelength and raising the frequency you hear.

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**Lower** pitch — the wavefronts stretch out, lengthening the wavelength and lowering the frequency you hear.

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**No** — the source always emits the same f. Only the **observed** frequency f' changes.

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$f' = f\left(\dfrac{v}{v \pm v_{s}}\right)$ — **minus** approaching, **plus** receding. **Given** in the data booklet.

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The observer meets the wavefronts **more often**, so the frequency heard is **higher**; the wavelength in the air is **unchanged**.

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Only a **moving source** squashes or stretches the wavelength. A moving observer just meets the crests at a different rate.

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The **minus** sign — it makes the denominator smaller, so f' is **larger** (higher pitch).

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The source moves forward between emitting each crest, so the **wavefronts ahead bunch together** → shorter wavelength → higher frequency.

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**High and flat** (approaching) → a **sharp step down** (passing) → **low and flat** (receding). Not a smooth slope.

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You hear it **above** its true pitch while it approaches, then a **sudden drop** to **below** its true pitch as it passes and recedes.

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Drawing the heard pitch **sliding down smoothly**. It actually steps **down sharply** at the instant the source passes.

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**Ahead** of the source, in the direction it moves: shorter wavelength, higher frequency.

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**No.** The speed of sound in the air stays the same; only the wavelength (moving source) or the rate of meeting crests (moving observer) changes.

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No: the drop is quick but **smooth**. It is a sudden step only when the motion is along the line through the listener.