Science · Class 9
Sound
Longitudinal waves, speed of sound, echo, and SONAR.
4 sections15–25 min read6 MCQs · 2 examples
Chapter Overview
Sound is a longitudinal mechanical wave requiring a medium. Topics include propagation, characteristics (pitch, loudness, quality), echo, speed of sound, SONAR, and structure of human ear.
Wave Properties
Wave speed
v = fλ
Time period
T = 1/f
Echo condition
Minimum distance ≈ 17 m (for 0.1 s persistence of hearing)
Characteristics
- Frequency (Hz) → pitch; higher f = higher pitch
- Amplitude → loudness; measured in decibel (dB)
- Quality/timbre distinguishes instruments at same pitch
- Audible range: 20 Hz – 20,000 Hz
Speed of Sound
- Fastest in solids, then liquids, slowest in gases
- v in air ≈ 344 m/s at 20°C
- Ultrasound: f > 20 kHz — used in imaging and SONAR
Solved Examples
Step-by-step solutions — read each step before checking the final answer.
Wavelength
Sound of frequency 512 Hz in air (v=340 m/s). Find λ.
- 1λ = v/f = 340/512 ≈ 0.66 m
Answer
≈ 0.66 m
Echo distance
Echo heard after 0.4 s. Speed of sound 340 m/s. Distance to cliff?
- 1Total path 2d = v×t = 136 m
- 2d = 68 m
Answer
68 m
Key Points to Remember
- ✓ Sound cannot travel in vacuum
- ✓ Compressions and rarefactions in longitudinal waves
- ✓ Eardrum vibrates → ossicles → cochlea
Exam Tips
- • Remember echo uses double distance
- • Distinguish pitch (frequency) and loudness (amplitude)
- • Draw longitudinal wave diagram
Practice MCQs — Sound
Test your understanding with topic-wise multiple choice questions. Explanations appear after each answer.
Question 1 of 6Score: 0/0
Sound is a ___ wave: