Quick Revision Notes: Class 8 Science

Chapter: Sound (ध्वनि)

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### 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.

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### 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.

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### 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.

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### 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).

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### 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).

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### 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.