Sound

• What is Sound?
  • Form of energy
  • Produced by vibrating bodies
  • Enables us to hear
• Production of Sound
  • Produced by a vibrating body (back-and-forth/to-and-fro motion)
  • Examples
    • Stretched rubber band
    • Beating a drum / membrane
    • Plucking a sitar string
    • Human vocal cords
• Propagation of Sound (How it travels)
  • Needs a medium (Solid, Liquid, or Gas)
  • Cannot travel through a vacuum (proven by bell jar experiment)
  • Travels in waves through particle vibration
• Sound Produced by Humans
  • Produced by the voice box or larynx
  • Two vocal cords stretched across the larynx
  • Narrow slit left for air passage
  • Lungs push air -> vocal cords vibrate -> sound produced
  • Differences in cords: Men (longer), Women (shorter), Children (very short)
• Hearing Sound through Ears
  • Outer part shaped like a funnel (pinna)
  • Sound travels down the ear canal
  • Hits a thin stretched membrane: eardrum (tympanum)
  • Eardrum vibrates -> passes vibrations to inner ear
  • Signals sent to the brain
• Characteristics of Sound Waves
  • Amplitude ($A$)
    • Maximum displacement from mean position
    • Unit: Decibel (dB) or meter
    • Determines Loudness
  • Frequency ($f$)
    • Number of oscillations per second
    • Unit: Hertz (Hz)
    • Determines Pitch (shrillness)
  • Time Period ($T$)
    • Time taken for one complete oscillation ($T = 1/f$)
• Amplitude and Loudness
  • Directly proportional to the square of amplitude ($\text{Loudness} \propto A^2$)
  • Higher amplitude = Louder sound
  • Lower amplitude = Faint sound
  • Measured in decibels (dB)
• Frequency and Pitch (Shrillness)
  • Higher frequency = Higher pitch (shrill sound, e.g., baby's voice, whistle)
  • Lower frequency = Lower pitch (grave/flat sound, e.g., lion's roar, man's voice)
• Audible and Inaudible Sounds
  • Audible range for humans: $20\text{ Hz}$ to $20,000\text{ Hz}$
  • Infrasonic (Inaudible): Frequency $< 20\text{ Hz}$ (e.g., whales, earthquakes)
  • Ultrasonic (Inaudible): Frequency $> 20,000\text{ Hz}$ (e.g., bats, dogs)
• Noise and Music
  • Music: Pleasing to the ear, regular/periodic vibrations
  • Noise: Unpleasant sounds, irregular/random vibrations
• Noise Pollution
  • Presence of excessive or unwanted sounds in the environment
  • Major sources: Vehicles, factories, loudspeakers, bursting crackers, appliances
  • Harmful Effects
    • Lack of sleep
    • Hypertension (high blood pressure)
    • Anxiety
    • Temporary or permanent hearing impairment
  • Measures to Limit Noise Pollution
    • Install silencers in vehicles and industrial machines
    • Operate TVs and music systems at low volumes
    • Plant trees along roads (green mufflers)
    • Use horn sparingly in public places