Doppler Effect: A Source Approaching a Listener

1 · Predict

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?

• 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.

2 · Set Up

1. Open the doppler-approach preset and press Reset. The source emits 440 Hz in 20°C air; the listener is stationary.
2. Enable the observed-frequency readout at the observer.
3. Set the source's approach speed for each trial and record the observed frequency.

3 · Collect Data

Plot observed frequency f′ (y-axis) against source speed v_s (x-axis) for your three trials.

4 · Analyze

1. 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.
2. 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.

5 · Extend

1. 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.
2. 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′.

The Physics Behind This Experiment

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