📝 Chapter Notes & Revision

Sound

🏫 MP BoardClass 8Science

📐 Formula & Cheat Sheet (English)

Quick Revision Notes: Class 8 Science

Chapter: Sound (ध्वनि)

### Concept 1: What is Sound? (ध्वनि क्या है?)

  • Definition: Sound is a form of energy that produces a sensation of hearing in our ears. It is produced by vibrating bodies.
  • Vibration (कंपन): To and fro or back and forth motion of an object is termed vibration.
  • Production of Sound: Sound needs a vibrating body to be produced. For example, the buzzing sound of a bee is produced by the vibration of its wings.

### Concept 2: Propagation of Sound (ध्वनि का संचरण)

  • Medium Requirement: Sound requires a material medium (solid, liquid, or gas) to travel.
  • Vacuum: Sound cannot travel through a vacuum because there are no particles to transmit the vibration.
  • Speed in Media: Sound travels fastest in solids, slower in liquids, and slowest in gases.
  • Human Voice: In humans, sound is produced by the voice box or the larynx (कנת / यंत्र). Two vocal cords stretched across the voice box vibrate to produce sound.

### Concept 3: How Do We Hear Sound? (हम ध्वनि कैसे सुनते हैं?)

  • Eardrum (कर्णपटह): The shape of the outer part of the ear is like a funnel. When sound enters it, it travels down a canal at the end of which is a thin stretched membrane called the eardrum.
  • Mechanism:
    1. Sound waves hit the eardrum.
    2. The eardrum vibrates.
    3. The vibrations are sent to the inner ear and then to the brain, and we hear the sound.

### Concept 4: Key Terms and Characteristics of Sound (ध्वनि के मुख्य लक्षण)

  1. Amplitude (आयाम):

    • The maximum displacement of a vibrating body from its mean position.
    • Unit: Meter (m)
    • Relation: Loudness depends on amplitude. Greater the amplitude, louder the sound ($Loudness \propto Amplitude^2$).

  2. Frequency (आवृति):

    • The number of oscillations per second.
    • Unit: Hertz (Hz)
    • Relation: Pitch (त्व/तीक्ष्णता) depends on frequency. Higher the frequency, higher is the pitch (shrillness), and vice versa.

  3. Time Period (आवर्तकाल):

    • The time taken for one complete oscillation.
    • Formula: Time Period (T) = 1 / Frequency (f)
    • Unit: Seconds (s)

  4. Loudness (प्रबलता):

    • Measured in decibels (dB).
    • Soft whisper: ~30 dB
    • Normal conversation: ~60 dB
    • Factory / Rock music: Above 80 dB (becomes noise/painful).

### Concept 5: Audible and Inaudible Sounds (श्रव्य तथा अश्रव्य ध्वनियाँ)

  • Audible Range (श्रव्य सीमा): Humans can hear sounds with frequencies between 20 Hz and 20,000 Hz.
  • Infrasonic Sound (अवध्वनि): Sounds of frequency less than 20 Hz. (e.g., produced by earthquakes, whales).
  • Ultrasonic Sound (पराध्वनि): Sounds of frequency greater than 20,000 Hz. (e.g., produced by bats, dolphins).

### Concept 6: Noise and Music (संगीत तथा शोर)

  • Music (संगीत): Pleasing to the ear, has a harmonious and regular effect.
  • Noise (शोर): Unpleasant sounds, unwanted and irregular.
  • Noise Pollution (ध्वनि प्रदूषण): Presence of excessive or unwanted sounds in the environment.
    • Major Causes: Vehicles, machinery, crackers, loudspeakers, construction work.
    • Harmful Effects: Lack of sleep, hypertension (high blood pressure), anxiety, temporary or permanent hearing impairment.
    • Measures to Control: Fitting silencers in vehicles, planting trees (green mufflers), turning down TV/radio volumes, setting up industries away from residential areas.