General Science Notes Chapter 15 Sound Class 8 Maharashtra Board

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.

4. Sound Needs a Medium to Travel

Experiment: Bell Jar Vacuum Test

1. An electric bell is placed inside a glass bell jar connected to a vacuum pump.
2. When the bell rings, the sound is clearly heard.
3. As air is removed, the sound becomes weaker.
4. When a vacuum is created, no sound is heard.
5. 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).

(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).

(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

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

1. SONAR (Sound Navigation and Ranging) – Used by ships and submarines to measure ocean depth.
2. Medical Ultrasound – Used to see images of internal body parts.
3. 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?

1. Use soundproofing in buildings.
2. Avoid honking unnecessarily.
3. Plant more trees, as they absorb sound.