Doppler effect: an approaching source
An ambulance with its siren on approaches a stationary listener. Compared with the frequency emitted, the frequency the listener hears is:
Answer C: explanation for every choice
A
Stretched wavefronts and lower frequency occur when the source moves away.
B
Relative motion between source and listener changes the observed frequency.
C
Correct. The source moves toward the listener between wavefronts, shortening the wavelength the listener receives.
D
The speed of sound depends on the medium, not on the source's motion.
TakeawayApproaching: higher pitch. Moving away: lower pitch.
See your set score →
Reviewed 2026-11-05 by a medical student. Spot an error? Report it with question ID cp27.