Doppler Effect Physics Example 1

Follow the full solution, then compare it with the other examples linked below.

Example 1

easy
An ambulance siren emits sound at 700 Hz700 \text{ Hz}. As the ambulance approaches you at 30 m/s30 \text{ m/s}, what frequency do you hear? Use vsound=340 m/sv_{\text{sound}} = 340 \text{ m/s}.

Solution

  1. 1
    Doppler effect for approaching source: f′=f×vv−vsf' = f \times \frac{v}{v - v_s}.
  2. 2
    f′=700×340340−30=700×340310f' = 700 \times \frac{340}{340 - 30} = 700 \times \frac{340}{310}
  3. 3
    f′=700×1.097≈768 Hzf' = 700 \times 1.097 \approx 768 \text{ Hz}

Answer

f′≈768 Hzf' \approx 768 \text{ Hz}
The Doppler effect causes the perceived frequency to increase when a sound source approaches and decrease when it moves away. This is why an ambulance siren sounds higher-pitched as it approaches.

About Doppler Effect

The change in the observed frequency (and wavelength) of a wave when the source and the observer are in relative motion.

Learn more about Doppler Effect →

More Doppler Effect Examples

Example 2 medium

A train whistle blows at [formula]. You are stationary and hear [formula]. Is the train approaching

Example 3 medium

A car horn emits [formula]. You drive toward the stationary car at [formula]. What frequency do you

Example 4 hard

Police radar emits microwaves at [formula]. The reflected signal from an approaching car has a frequ

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