Doppler Frequency Shift
Doppler Frequency Shift
The Doppler effect raises the pitch of an approaching source and lowers it as it recedes — heard as a passing siren.
For an approaching source f_obs = f·c/(c − v_s). where f is the source frequency, c the wave speed and v_s the source speed.
Inputs
f := 700 Hz=700 Hz
c := 340 m/s=340 m/s
v_s := 30 m/s=30 m/s
Apply the Doppler formula for an approaching source.
f_obs := f·c/(c - v_s)=767.74 Hz
Results
The approaching siren sounds higher than 700 Hz; once it passes, the sign of v_s flips and the pitch drops. The same physics powers radar and ultrasound flow meters.
Assumptions & limitations
- Source moving toward a stationary observer.
- Speeds below the wave speed.
- Still medium.