Doppler effect for sound: Class 11 practice
When a source moves towards a still listener, the observed frequency rises: f′ = f·v/(v − vₛ); moving away, f′ = f·v/(v + vₛ). Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Wave speed
The idea behind doppler effect for sound
When a source moves towards a still listener, the observed frequency rises: f′ = f·v/(v − vₛ); moving away, f′ = f·v/(v + vₛ). For a moving listener and still source, f′ = f(v ± vₒ)/v.
Step-by-step method
- Decide who moves and in which direction.
- Choose the sign that raises f when approaching.
- Substitute v (≈ 340 m/s).
Worked example 1 (Basic)
A 900 Hz siren approaches a still listener at 44 m/s (v = 340 m/s). What frequency is heard?
- f′ = 900 × 340/(340 − 44)
- Answer: 1033.78 Hz
Worked example 2 (Standard)
A 400 Hz source moves away from a still listener at 41 m/s. What frequency is heard?
- f′ = 400 × 340/(340 + 41)
- Answer: 356.96 Hz
Worked example 3 (Challenge)
A listener runs at 29 m/s towards a still 1200 Hz source. What frequency is heard?
- f′ = 1200 × (340 + 29)/340
- Answer: 1302.35 Hz
Common mistakes and misconceptions
- Wrong sign
- Using the formula for a moving listener when the source moves
Practice questions
- (Basic) A 1200 Hz siren approaches a still listener at 36 m/s (v = 340 m/s). What frequency is heard?
- (Basic) A 900 Hz siren approaches a still listener at 29 m/s (v = 340 m/s). What frequency is heard?
- (Standard) A 300 Hz source moves away from a still listener at 24 m/s. What frequency is heard?
- (Standard) A 1100 Hz source moves away from a still listener at 41 m/s. What frequency is heard?
- (Challenge) A listener runs at 29 m/s towards a still 1000 Hz source. What frequency is heard?
- (Challenge) A listener runs at 15 m/s towards a still 600 Hz source. What frequency is heard?
Answer key with full working
- Q1: f′ = 1200 × 340/(340 − 36) → Answer: 1342.11 Hz
- Q2: f′ = 900 × 340/(340 − 29) → Answer: 983.92 Hz
- Q3: f′ = 300 × 340/(340 + 24) → Answer: 280.22 Hz
- Q4: f′ = 1100 × 340/(340 + 41) → Answer: 981.63 Hz
- Q5: f′ = 1000 × (340 + 29)/340 → Answer: 1085.29 Hz
- Q6: f′ = 600 × (340 + 15)/340 → Answer: 626.47 Hz
Syllabus note
This skill is listed under "Waves" for Class 11 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is doppler effect for sound in?
"Waves", Class 11 Physics (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.