Quick answer: Sound is a short chapter with easy numericals on v = fλ and echo, and definition questions on ultrasound and SONAR. It is 4–5 marks and quick to master.
Nature of sound
- Produced by vibrating bodies; needs a medium (no sound in vacuum).
- Longitudinal wave: compressions (high pressure) and rarefactions (low pressure).
- Speed: solids > liquids > gases; in air ≈ 344 m/s at 22°C; increases with temperature.
Wave terms
| Term | Meaning | Unit |
|---|---|---|
| Wavelength λ | Distance between two consecutive compressions | m |
| Frequency f | Vibrations per second; decides pitch | Hz |
| Time period T | 1/f | s |
| Amplitude | Max displacement; decides loudness | m |
| Speed | v = fλ | m/s |
Echo and reverberation
- Echo needs the reflected sound to reach after 0.1 s → minimum distance = 344 × 0.1/2 = 17.2 m.
- Reverberation: persistence due to repeated reflections; reduced by sound-absorbing materials in halls.
Hearing and ultrasound
- Audible range 20 Hz – 20,000 Hz; infrasound below, ultrasound above.
- Ultrasound uses: cleaning parts, detecting cracks in metal blocks, echocardiography, ultrasonography, breaking kidney stones.
- SONAR: distance = speed × time/2.
Numericals
1. f = 500 Hz, v = 340 m/s → λ = 0.68 m.
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2. A ship sends a pulse and receives the echo after 3 s; speed in water 1500 m/s → depth = 1500 × 3/2 = 2250 m.
3. Thunder heard 5 s after lightning (v = 340) → cloud is 1700 m away.
4. A wave of 2 m wavelength and speed 400 m/s has f = 200 Hz, T = 0.005 s.
Common mistakes
- Forgetting to halve the distance in echo/SONAR problems
- Mixing loudness (amplitude) with pitch (frequency)
- Calling sound a transverse wave
Related: Gravitation Class 9.
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Frequently asked questions
Are these Sound Class 9 Notes notes enough for the exam?
They cover every formula, definition and question type that appears in school and board papers for this chapter. Use them with the NCERT examples, exercise questions and the last five years' papers to be fully prepared.
How should I revise Sound Class 9 Notes?
Revise the notes after 1 day, 7 days and 30 days. Solve five questions each time instead of only reading, and keep an error list of the steps you get wrong.
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