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Sound Notes with PDF

By October 4, 2025 Educational Resources 0 Comments

These notes are your ultimate revision weapon to revise Sound . We’ve distilled years of previous exam questions (PYQs) into one powerful, concise resource. Everything you need to know, nothing you don’t.

  • PYQs, Decoded: All key concepts from past exams, organized and simplified.
  • Revise in Record Time: Short, precise, and designed for last-minute review.
  • Focus on What Matters: Master high-probability topics and boost your confidence.
  • Free PDF to download.

Sound: Fundamental Principles

1. Nature of Sound Waves

  • Definition: Sound is a mechanical wave that requires a material medium to travel.
  • Wave Types:
    • In gases, liquids, and plasma: Longitudinal waves.
    • In solids: Can be both longitudinal and transverse waves.
  • Cannot Travel in a Vacuum: It requires particles to vibrate. (e.g., sound cannot travel on the moon).

2. Characteristics of Sound

  • Pitch: Determines how high or low a sound is.
    • Depends solely on frequency.
    • Higher frequency = Higher pitch.
    • Fact Check: A shorter flute (shorter air column) produces a higher frequency, not a lower one.
  • Loudness (Intensity): Determines the volume of a sound.
    • Depends on the amplitude of the sound wave.
    • Unit of Measurement: Decibel (dB).
  • Quality (Timbre): The characteristic that allows us to distinguish between the same note played on different instruments (e.g., a sitar vs. a flute), even when the pitch and loudness are the same.

3. Behavior of Sound Waves

  • When Changing Medium:
    • Frequency remains constant.
    • Speed and wavelength change.
  • Reflection:
    • Echo: A reflected sound heard separately from the original.
      • To hear a clear echo, the minimum distance to the reflecting surface is ~17 meters (allowing a time gap of ~1/10th of a second).
    • Reverberation: The effect of multiple, fast sound reflections in an enclosed space (e.g., a church).
    • Application: Concert hall walls are designed to absorb sound to prevent echoes and reverberation for clarity.
  • Loudness decreases as the sound travels away from the source due to energy spreading out.

Speed of Sound

1. Dependence on the Medium

  • General Rule: Solid > Liquid > Gas.
  • Reason: Depends on the elasticity and density of the medium.
  • Examples:
    • Gas (Slowest): Nitrogen / Air ≈ 343 m/s at 20°C.
    • Liquid (Faster): Seawater ≈ 1531 m/s.
    • Solid (Fastest): Iron ≈ 5950 m/s.
  • Order of Speed: Nitrogen (Gas) < Water (Liquid) < Steel (Solid). Air < Water < Steel.
  • Comparison: Speed is greater in aluminum than in glass. Faster through a steel rail than through air.

2. Dependence on Temperature

  • The speed of sound increases with an increase in temperature.
  • At higher altitudes (e.g., 15 km) where the temperature is lower, the speed decreases.

3. Measurement: Mach Number

  • The Mach Number is the ratio of an object’s speed to the speed of sound in the surrounding medium.
  • Mach 1 means the object is traveling at the speed of sound (supersonic).

Classification by Frequency

1. Audible Sound

  • Range: 20 Hz to 20,000 Hz (the typical range of human hearing).

2. Ultrasonic Sound

  • Definition: Sound with a frequency greater than 20,000 Hz. Humans cannot hear it.
  • Applications:
    • Medical: Sonography/Ultrasonography (imaging internal body structures, monitoring pregnancy).
    • Cleaning: Ultrasonic cleaners (for clothes, objects).
    • Detection & Ranging: SONAR (detecting submarines, measuring sea depth), detecting aircraft, controlling automatic doors.
    • Other: Destroying insects.
  • Biological Use: Bats use ultrasonic waves for echolocation (navigation and hunting).

3. Infrasonic Sound

  • Biological Example: The vibration of the heart produces sound in the infrasonic range.

Sound Measurement and Impact

1. Sound Levels (Decibels – dB)

  • Comfortable Level: ~60 dB (Optimum for humans).
  • Tolerable Limit: ~85 dB.
  • Hazardous Pollution: Sound above 80 dB is considered hazardous noise pollution.
  • WHO Safe City Level: 45 dB.
  • Common Examples:
    • Rustling leaves: ~20 dB
    • Normal conversation: ~30-60 dB
    • Machine shop: ~100 dB

2. Environmental Impact

  • Supersonic jets release nitrogen oxides at high altitudes, contributing to the depletion of the ozone layer.

Applications and Specific Contexts

1. Music

  • Musical Scale: The diatonic scale consists of 8 notes (an octave).
  • Frequency Ratio: The ratio between the first and last note (the octave) is 2:1.
    • Example: If ‘Sa’ (C) = 256 Hz, the next ‘Sa’ (C1) = 512 Hz. The note ‘Ni’ (B) just before it is 480 Hz.

2. Medical Technology

  • Stethoscope: Works through the multiple reflections of sound waves inside its tube.

3. Television Broadcast Technology

  • Analog TV:
    • Picture (Video): Transmitted using Amplitude Modulation (AM).
    • Audio (Sound): Transmitted using Frequency Modulation (FM).
  • Modern TVs: Audio and video start simultaneously.

4. Linguistics

  • Phonemes: The basic, smallest units of sound in a language. They have no meaning themselves, but changing one can change the meaning of a word.

Know More About Sound:

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