The Doppler Effect Practice Test

A stationary siren emits a steady tone in still air. Two observers run: Observer A runs toward the siren; Observer B runs away from it, each at the same speed. Compared to Observer B, the frequency heard by Observer A is

A stationary buzzer emits a constant tone. An observer moves directly away from the buzzer at constant speed. Compared to when the observer is at rest, the observed frequency is