Waves & Sound
Doppler — approaching source
Press Play — raise source v_s toward the observer; f′ rises above 440 Hz.
Hear and visualize the Doppler effect as a moving sound source approaches a stationary observer. Measure apparent frequency shift and connect to relative motion.
Doppler effect
A moving source compresses wavefronts ahead of it, raising the observed frequency.
When the source approaches a stationary observer, each successive crest is emitted from a closer position, arriving more frequently than the emitted frequency.
- f′ = f · c/(c − v_s)
- Approaching → higher f′
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
A sound source moves toward a stationary listener while emitting a steady tone. Does the listener hear a higher, lower, or the same pitch as the source's true frequency?
Predictions to weigh
- The listener hears a higher pitch than the source's true frequency.
- The listener hears the exact same pitch.
- The listener hears a lower pitch.
Variable roles
What you set:
- Source frequency f (Hz)
- Source speed v_s (m/s)
What you measure:
- Observed frequency f′ (Hz)
How the investigation runs
- Open the doppler-approach preset and press Reset. The source emits 440 Hz in 20°C air; the listener is stationary.
- Enable the observed-frequency readout at the observer.
- Set the source's approach speed for each trial and record the observed frequency.
Governing equation
Doppler Effect (moving source) — f′ = f·(c + v_o)/(c − v_s)
A source moving toward a stationary listener compresses the wavefronts ahead of it, raising the frequency the listener hears: f′ = f·c/(c − v_s).
What the printable worksheet asks students to work out
- For one trial, compute f′ = f·c/(c − v_s) using c = 331.3·√(1 + 20/273.15) m/s for 20°C air. Compare to the table.
- Explain why f′ keeps increasing as v_s grows, and what would happen to the formula (and to the sound) if v_s ever reached the speed of sound c.
Where this shows up beyond the lab
- An ambulance siren sounds higher-pitched as it approaches and suddenly drops in pitch as it passes and recedes. Use the Doppler formula's sign convention to explain the drop.
- This experiment only covers an approaching source. If the source were moving away instead, would you expect (c − v_s) to become (c + v_s) in the denominator? Explain what that does to f′.
- AP Physics 2 — Unit 14: Waves, Sound, and Physical Optics
- IB Physics — C.5 Doppler effect
- General High School Physics — Waves & sound
- NGSS High School Physics — Wave properties
- Welcome to Doppler Approach
- Select the observer
- Press Play
- Rising pitch
- Open the Properties panel
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.