Sound is a very "observable" topic. You encounter it every day. For CBSE and competitive exams, the questions here are usually based on Speed, Frequency ranges, and Applications (SONAR/Ultrasound).

1. Production and Propagation

• Source: Sound is produced by Vibration. No vibration = No sound.
• Nature: Sound is a Mechanical Wave. It needs a material medium (Solid, Liquid, or Gas).
  • Exam Fact: Sound cannot travel in a Vacuum. (Experiment: Bell jar experiment).
• Type of Wave: Longitudinal Wave.
  • Particles oscillate back and forth parallel to the direction of propagation.
  • Structure: Compressions (High Pressure) and Rarefactions (Low Pressure).
  • Contrast: Light is a Transverse Wave (Particles oscillate perpendicular).

2. Characteristics of Sound (The Jargon)

You must understand these four terms to solve conceptual questions.

1. Frequency (ν - Nu)

Number of oscillations per second. Determines Pitch.
High Frequency: High Pitch/Shrill voice (e.g., woman's voice, whistle).
Low Frequency: Low Pitch/Deep voice (e.g., lion's roar).
Unit: Hertz (Hz).

2. Amplitude (A)

The magnitude of disturbance. Determines Loudness.
High Amplitude: Loud sound.
Mentor’s Note: Loudness is measured in Decibels (dB).

3. Time Period (T) & 4. Wavelength (λ)

Time Period: Time taken for one complete oscillation. (T = 1/ν).
Wavelength (λ): The distance between two consecutive Compressions or Rarefactions.

3. Speed of Sound (High Yield Topic)

The speed depends on the properties of the medium.

• Order of Speed: Solids > Liquids > Gases.
  Reason: Solids are denser and more elastic, allowing energy to transfer faster.
• Effect of Temperature: Speed of sound increases with temperature.
  Reason: Heat adds kinetic energy to particles, making them vibrate faster.

4. Reflection of Sound

Just like light, sound bounces off surfaces.

Echo

The distinct repetition of sound.
Condition: Time gap between original and reflected sound must be at least 0.1 seconds.
Minimum Distance: Since speed ≈ 344m/s, the obstacle must be at least 17.2 meters away.

Reverberation

Multiple reflections causing sound to persist (e.g., in a big empty hall).
Fix: Use sound-absorbent materials (curtains, carpets).

Applications of Multiple Reflection

• Stethoscope: Sound travels via multiple reflections in the tube.
• Megaphones: Designed to guide sound forward.
• Curved Ceilings: Reflect sound to the audience in concert halls.

5. Range of Hearing

• Audible Range (Humans): 20 Hz to 20,000 Hz (20 kHz).
• Infrasonic (Infrasound): Below 20 Hz.
  Users: Whales, Elephants, Rhinoceroses. Earthquakes produce infrasound before the main shock.
• Ultrasonic (Ultrasound): Above 20,000 Hz.
  Users: Bats, Dolphins (for hunting/navigation).

6. Applications of Ultrasound (Technology)

• Cleaning: Spiral tubes/electronic components (Vibration shakes off dust).
• Metal Testing: Detects cracks (waves reflect back from the crack).
• Medical:
  • Echocardiography: Image of the heart.
  • Ultrasonography: Images of internal organs (liver, fetus).
  • Lithotripsy: Breaking kidney stones into grains.
• SONAR (Sound Navigation And Ranging):
  Used by ships to measure ocean depth.
  Formula: 2d = v × t (Distance doubled as sound goes down and comes back).

7. The Human Ear (Brief Overview)

• Pinna (Outer Ear): Collects sound.
• Eardrum (Tympanic Membrane): Vibrates when sound hits it.
• Middle Ear Bones: Hammer, Anvil, Stirrup (Amplify vibrations).
• Cochlea (Inner Ear): Converts vibrations into electrical signals for the brain.

8. Mentor’s Final Drill (Exam-Ready Questions)

Study Strategy: Focus on Units (Hz, dB) and Applications (SONAR, Ultrasound). Don't worry too much about the complex math of waves, but remember the relationship: v = λν.