Science · Class 8
Sound
Sound production, propagation, pitch, loudness, and ultrasound.
4 sections15–25 min read6 MCQs · 2 examples
Chapter Overview
Sound is produced by vibrating objects and travels as longitudinal waves through a medium. This chapter covers propagation, human ear, amplitude, frequency, pitch, loudness, and noise pollution.
Production and Propagation
- Vibrating strings, membranes, vocal cords produce sound
- Needs medium — cannot travel in vacuum
- Longitudinal wave: compressions and rarefactions
Characteristics
- Amplitude: loudness — larger amplitude = louder
- Frequency: pitch — higher frequency = shriller
- Time period T = 1/frequency; unit hertz (Hz)
- Audible range: 20 Hz – 20,000 Hz
Noise Pollution
- Unpleasant loud sound harms hearing
- Reduce: silencers, plantation, limited hours
- Eardrum vibrates — damage from very loud noise
Solved Examples
Step-by-step solutions — read each step before checking the final answer.
Vacuum bell jar
Why does sound not travel in vacuum?
- 1No particles to vibrate and transfer energy
- 2Bell jar experiment: sound fades as air pumped out
- 3Needs material medium
Answer
No medium for wave propagation
Pitch vs loudness
Distinguish pitch and loudness.
- 1Pitch: how shrill — depends on frequency
- 2Loudness: how loud — depends on amplitude
- 3High pitch ≠ necessarily loud
Answer
Pitch → frequency; Loudness → amplitude
Key Points to Remember
- ✓ Sound travels fastest in solids, slowest in gases
- ✓ Ultrasound above 20 kHz — used in imaging
- ✓ Echo: reflected sound heard again
Exam Tips
- • Describe human ear with diagram
- • Why do musical instruments have sound boxes?
- • List ways to reduce noise pollution
Practice MCQs — Sound
Test your understanding with topic-wise multiple choice questions. Explanations appear after each answer.
Question 1 of 6Score: 0/0
Sound needs ___ to travel.