Chapter: Sound

• Nature of Sound

  • Form of energy
  • Produces sensation of hearing in our ears
  • Law of conservation of mass and energy applies

• Production of Sound

  • Produced by vibration (rapid to and fro motion)
  • Example: Vibrating tuning fork, striking a drum, vocal cords

• Propagation of Sound

  • Requires a medium (solid, liquid, or gas) to travel
  • Cannot travel through vacuum (proven by bell jar experiment)
  • Moves in the form of waves

• Waves and Wave Motion

  • Mechanical Waves: Need material medium for propagation
  • Longitudinal Waves: Particles vibrate parallel to wave direction
    • Consists of Compressions (high pressure/density)
    • Consists of Rarefactions (low pressure/density)
  • Transverse Waves: Particles vibrate perpendicular to wave direction (e.g., light waves)

• Characteristics of a Sound Wave

  • Frequency ($v$): Number of oscillations per unit time
    • Unit: Hertz (Hz)
  • Time Period ($T$): Time taken for one complete oscillation
    • Formula: $T = 1/v$
  • Amplitude ($A$): Maximum displacement of particles from mean position
    • Determines loudness
  • Wavelength ($\lambda$): Distance between two consecutive compressions or rarefactions
    • Unit: Metre (m)
  • Speed ($v$): Distance travelled per unit time
    • Formula: $v = \lambda \times v$

• Speed of Sound in Different Media

  • Depends on temperature and medium properties
  • Order of speed: Solids > Liquids > Gases
  • Speed in air: Approx. 343 m/s at $20^\circ\text{C}$
  • Sonic Boom: Speed of an object exceeds the speed of sound (supersonic)

• Reflection of Sound

  • Follows the laws of reflection
  • Echo: Repetition of sound due to reflection from a rigid obstacle
    • Minimum distance required: 17.2 metres
    • Persistence of hearing: $1/10\text{th}$ of a second
  • Reverberation: Persistence of sound in a big hall due to repeated reflections
    • Minimized by using sound-absorbing materials (curios, compressed fibreboard)

• Applications of Reflection of Sound

  • Megaphones and loudhailers (horn-shaped tubes)
  • Stethoscope (multiple reflections of sound to reach doctor's ears)
  • Curved ceilings of concert halls and conference halls
  • SONAR (Sound Navigation And Ranging)
    • Measures depth of sea and locates underwater objects
    • Uses ultrasonic waves

• Range of Hearing

  • Audible Range for humans: $20\text{ Hz}$ to $20,000\text{ Hz}$
  • Infrasonic Sound: Frequency $< 20\text{ Hz}$ (produced by whales, elephants, earthquakes)
  • Ultrasonic Sound: Frequency $> 20,000\text{ Hz}$ (produced by bats, dolphins, porpoises)

• Structure of Human Ear

  • Outer Ear (Pinna): Collects sound waves
  • Middle Ear: Eardrum amplifies vibrations through three bones (hammer, anvil, stirrup)
  • Inner Ear: Cochlea converts pressure variations into electrical signals
  • Auditory Nerve: Sends electrical signals to the brain