Notes For All Chapters – General Science Class 8
1. Introduction
- Sound is a form of energy that is produced by vibrations.
- It travels in the form of waves through a medium like air, water, or solids.
- Without a medium (like in space), sound cannot travel.
2. Production of Sound
Sound is produced when an object vibrates.
Vibrations create compressions and rarefactions, forming a sound wave.
Examples of sound production:
- Drums vibrate when hit.
- Strings of a guitar vibrate when plucked.
- Human vocal cords vibrate to produce speech.
3. Propagation of Sound (How Sound Travels)
- Sound travels as a longitudinal wave, which means the particles of the medium vibrate back and forth in the direction of the wave.
- Important Terms:
- Compression: Region where air molecules are close together (high pressure).
- Rarefaction: Region where air molecules are far apart (low pressure).
- Sound travels faster in solids than in liquids and gases because particles are closely packed.
| Medium | Speed of Sound (m/s) |
|---|---|
| Air | 340 m/s |
| Water | 1500 m/s |
| Iron | 5000 m/s |
4. Sound Needs a Medium to Travel
Experiment: Bell Jar Vacuum Test
- An electric bell is placed inside a glass bell jar connected to a vacuum pump.
- When the bell rings, the sound is clearly heard.
- As air is removed, the sound becomes weaker.
- When a vacuum is created, no sound is heard.
- Conclusion: Sound cannot travel in a vacuum; it needs a medium like air, water, or solids.
5. Characteristics of Sound
(A) Loudness
- Definition: The amount of energy in a sound wave.
- Depends on: The amplitude of the wave (higher amplitude = louder sound).
- Unit: Decibel (dB).
| Sound Source | Loudness (dB) |
|---|---|
| Whisper | 30 dB |
| Normal Conversation | 60 dB |
| Traffic Noise | 80 dB |
| Jet Engine | 120 dB (can cause hearing damage) |
(B) Pitch
- Definition: How high or low a sound is.
- Depends on: The frequency of the sound wave (higher frequency = higher pitch).
- Unit: Hertz (Hz).
| Sound Source | Pitch (Hz) |
|---|---|
| Man’s Voice | 120 Hz |
| Woman’s Voice | 250 Hz |
| Dog’s Bark | 500 Hz |
(C) Quality (Timbre)
- Definition: The uniqueness of a sound that helps us identify different voices or musical instruments.
- Example: A guitar and violin may play the same note, but they sound different due to timbre.
6. Human Voice and Sound Production
- In humans, sound is produced in the larynx (voice box).
- The vocal cords vibrate when air passes through them, creating sound.
- Changing the tension of the vocal cords changes the pitch of sound.
- Tighter cords → Higher pitch (women, children).
- Looser cords → Lower pitch (men).
7. Sound in Musical Instruments
| Instrument Type | How Sound is Produced | Examples |
|---|---|---|
| String Instruments | Vibrations of stretched strings | Guitar, Violin, Sitar |
| Wind Instruments | Vibrations of air column | Flute, Trumpet, Shehnai |
| Percussion Instruments | Vibrations of membranes | Drums, Tabla, Dholak |
8. Reflection of Sound (Echo)
- Echo: The repetition of sound when it reflects off a surface.
- Example: Shouting in a large hall or near a mountain produces an echo.
- To hear an echo, the reflected sound must reach us after 0.1 seconds.
- Distance required for an echo: At least 17 meters (34 meters for round trip).
9. Uses of Echo
- SONAR (Sound Navigation and Ranging) – Used by ships and submarines to measure ocean depth.
- Medical Ultrasound – Used to see images of internal body parts.
- Bats use echo to find food and navigate in the dark.
10. Noise and Noise Pollution
(A) What is Noise?
- Unpleasant and unwanted sound is called noise.
- Examples: Loudspeakers, vehicle horns, factory sounds.
(B) What is Noise Pollution?
- Excessive and harmful noise in the environment is called noise pollution.
- Effects of Noise Pollution:
- Hearing loss.
- Lack of sleep and stress.
- Damage to heart and brain.
(C) How to Reduce Noise Pollution?
- Use soundproofing in buildings.
- Avoid honking unnecessarily.
- Plant more trees, as they absorb sound.
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