PRELIMS

Sound Waves

Nature and Categories of Sound

Sound waves are longitudinal mechanical waves. They are categorized by frequency:

1. Audible Waves: 20 Hz to 20,000 Hz. Sensitive to the human ear.
2. Infrasonic Waves: Frequencies less than 20 Hz.
   • Sources: Earthquakes, volcanic eruptions, ocean waves, elephants, whales.
3. Ultrasonic Waves: Frequencies greater than 20,000 Hz. Detectable by dogs, cats, bats, and mosquitoes.
   • Production: Galton's whistle, Hartman's generator, Piezo-electric effect (quartz crystal), or Magnetostriction.

Applications of Ultrasonic Waves

1. Sending signals.
2. Measuring the depth of the sea (SONAR).
3. Cleaning clothes, aeroplanes, and machinery parts of clocks.
4. Removing lamp-shoot from factory chimneys.
5. Sterilizing liquids.
6. Ultrasonography.

Speed of Sound

Speed depends on the elasticity and density of the medium.

• Medium Hierarchy: Speed is maximum in solids and minimum in gases.
• Properties:
  1. When entering a new medium, speed and wavelength change, but frequency remains unchanged.
  2. Speed is independent of frequency.
  3. Speed is independent of pressure.
  • Supersonic: Mach > 1.
  • Hypersonic: Mach > 5.
  • Subsonic: Mach < 1.
• Shock Waves: Conical disturbance produced by supersonic objects (carries huge energy, can damage buildings).
• Bow Waves: Produced by motorboats traveling faster than surface water waves.

Doppler Effect

• Definition: Apparent change in the frequency (and pitch) of sound due to relative motion between the source and the observer.
  • Approaching: Apparent frequency increases (pitch rises).
  • Receding: Apparent frequency decreases (pitch falls).
• Everyday Example: Change in pitch of a train's whistle or an ambulance siren as it passes by.
• Applications: Used in SONAR, RADAR, tracking satellites, and estimating the speed of stars/galaxies (red shift) and vehicles (traffic police speed guns).

Loudness, Decibel Scale and Noise Pollution

• Loudness: Sensation of intensity of sound perceived by the ear; measured on the decibel (dB) scale (logarithmic).
• Threshold of Hearing: 0 dB. Normal conversation: ~60 dB.
• Permissible Limit: 85 dB (prolonged exposure above this can cause hearing damage).
• Painful/Damaging: Above 120 dB (jet engine, firecrackers) can cause immediate ear damage.
• Noise Pollution: Ambient Air Quality Standards (CPCB) prescribe 55 dB (day) / 45 dB (night) for residential areas, and 75 dB (day) / 70 dB (night) for industrial areas.

Confused Pairs: Infrasonic vs Ultrasonic

• Infrasonic: Frequency < 20 Hz; produced by large, slow-vibrating sources (earthquakes, volcanoes); detected by elephants, whales.
• Ultrasonic: Frequency > 20,000 Hz; produced by small, fast-vibrating sources (quartz crystals); detected by bats, dogs, dolphins.
• Common Trap: Both are inaudible to humans, but for opposite reasons (too low vs too high frequency) — do not confuse with "silent."

UPSC Relevance

Sound properties are high-priority for conceptual questions.

• Speed factors: The independence of pressure and the 0.61 m/s temperature rule are common MCQ targets.
• Medical/Industrial S&T: Applications of ultrasonics (imaging/cleaning) and the frequency limits for various animals.
• Supersonic flight: Mach numbers and the physical impact of shock waves.