alleryTwo identical sirens sound, one on a parked car and one on a car speeding towards you. What differs?
AOnly the speeding siren is audible
BBoth are heard at the same pitch
CThe parked siren sounds higher
DThe speeding siren sounds higher
Answer & Solution
Correct answer: D. The speeding siren sounds higher
1. A stationary source sends the same wavelength to observers on every side.
2. A source moving towards you bunches its compressions closer together.
3. That shorter wavelength is received as a higher frequency, so a higher pitch.
4. Once it passes, the same siren drops to a lower pitch as the compressions spread out.
_Source: OpenStax Physics (high school, CC BY 4.0), Ch 14 "Sound", section 14.3 Doppler Effect and Sonic Booms_
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