Question

Doppler Effect-How would the pitch sound as car approaches woman? After passes her?A:

Answer 1

Short answer: as the car approaches the woman she hears a higher pitch than the emitted pitch; as it passes and moves away she hears a lower pitch.

Why: the Doppler effect. For a stationary observer and a source moving at speed vs along the line to the observer:
- approaching: f' = f · v / (v − vs)
- receding: f' = f · v / (v + vs)
where f is the emitted frequency and v is the speed of sound (~343 m/s at 20 °C).

Example: siren f = 700 Hz, car vs = 30 m/s:
- approaching: f' ≈ 343/(343−30)·700 ≈ 767 Hz (higher)
- after passing (receding): f' ≈ 343/(343+30)·700 ≈ 643 Hz (lower)

Behavior: the pitch changes continuously as the relative radial velocity changes and jumps from higher to lower as the car passes directly by. The sound also gets louder as it approaches and quieter as it recedes. (If the source were supersonic, you’d get a sonic boom instead of the ordinary Doppler shift.)