Waves, Sound & the Doppler Effect is one of 10 topics in the Grade 12 Physical Sciences CAPS syllabus, covered across 3 lessons on Study Vault. Below are real practice questions from this topic with full worked answers.
Sample practice questions
Easy
1. An ambulance siren sounds higher-pitched than normal as the ambulance approaches you, and lower-pitched as it drives away. What is this an example of?
The Doppler effect
Diffraction
Constructive interference
Resonance
Answer: The Doppler effect — This is the classic Doppler effect scenario: relative motion between the source (siren) and the listener (you) changes the observed frequency, even though the siren itself emits a constant frequency.
Easy
2. A source of sound moves toward a stationary listener. Compared to the frequency the source actually emits, the listener hears a frequency that is:
Higher
Lower
Exactly the same
Impossible to determine
Answer: Higher — When a source moves toward a listener, wavefronts are compressed in front of it, so the listener detects more wavefronts per second -- a higher observed frequency (pitch).
Medium
3. A listener moves away from a stationary sound source at constant speed. What happens to the frequency the source itself is emitting, and what happens to the frequency the listener detects?
The emitted frequency stays the same; the detected frequency decreases
The emitted frequency decreases; the detected frequency stays the same
Both the emitted and detected frequencies decrease
Both the emitted and detected frequencies increase
Answer: The emitted frequency stays the same; the detected frequency decreases — The source keeps emitting its own fixed frequency throughout -- only the frequency detected by the moving listener changes, because moving away means the listener encounters wavefronts less often.
