Class 8 Notes

Class 8 Science Notes Chapter 13 (Sound) – Science Book

Ajay Mehra

— 4 min read

Science
Explore detailed notes for Chapter 13 'Sound' from the NCERT Class 8 Science textbook, tailored for your exam preparation, followed by 10 Multiple Choice Questions to test your retentions power. Enjoy Learning :)

NCERT Class 8 Science - Chapter 13: Sound - Detailed Notes

1. Sound Production:

  • Fundamental Principle: Sound is produced by vibrating objects. Vibration is the rapid to and fro or back and forth motion of an object.
  • Examples:
    • Stretched rubber band when plucked.
    • School bell when struck.
    • Tuning fork when hit.
    • Diaphragm of a drum (tabla, dholak) when beaten.
    • Strings of a musical instrument (sitar, guitar) when plucked.
    • Air column in wind instruments (flute, trumpet) when air is blown.
  • Human Voice: Sound is produced by the larynx or voice box, located at the upper end of the windpipe.
    • Two vocal cords are stretched across the voice box, leaving a narrow slit for air passage.
    • When lungs force air through the slit, the vocal cords vibrate, producing sound.
    • Muscles attached to vocal cords can make them tight/thin (higher frequency/pitch) or loose/thick (lower frequency/pitch).

2. Propagation of Sound:

  • Medium Required: Sound needs a medium (solid, liquid, or gas) to travel from the source to the listener. Sound cannot travel through a vacuum (absence of matter).
  • Mechanism: Vibrating objects cause particles of the medium around them to vibrate. These vibrations are passed on from one particle to the next, creating a wave that travels through the medium. The particles themselves do not travel far, but the disturbance does.
  • Speed of Sound: Sound travels at different speeds in different media.
    • Generally, Speed of Sound: Solids > Liquids > Gases.
    • Sound travels fastest in solids, slower in liquids, and slowest in gases.
    • Example: Speed of sound in air is approx. 343 m/s (at 20°C), in water approx. 1480 m/s, and in steel approx. 5960 m/s.

3. Human Ear - How We Hear:

  • Structure: The ear has three main parts:
    • Outer Ear: Consists of the pinna (collects sound waves) and the ear canal. At the end of the ear canal is the eardrum (tympanic membrane).
    • Middle Ear: Contains three tiny, interconnected bones (hammer, anvil, stirrup). These bones amplify the vibrations.
    • Inner Ear: Contains the cochlea, a fluid-filled, spiral-shaped structure. The cochlea converts pressure variations (vibrations) into electrical signals.
  • Process of Hearing:
    1. Sound waves enter the ear canal via the pinna.
    2. Waves strike the eardrum, causing it to vibrate.
    3. Vibrations are amplified and transmitted through the middle ear bones (hammer, anvil, stirrup) to the inner ear.
    4. In the cochlea, the vibrations cause movement in the fluid and stimulate tiny hair cells.
    5. These hair cells convert the vibrations into electrical signals.
    6. The auditory nerve carries these signals to the brain.
    7. The brain interprets these signals as sound.

4. Characteristics of Sound Waves:

  • Oscillation: One complete to and fro motion of a vibrating object.
  • Frequency: The number of oscillations (or vibrations) per second.
    • Unit: Hertz (Hz).
    • 1 Hz = 1 oscillation per second.
    • Determines the pitch or shrillness of a sound.
  • Amplitude: The maximum displacement or distance moved by a point on a vibrating body or wave measured from its equilibrium position.
    • Determines the loudness of a sound.
    • Larger amplitude = Louder sound.
  • Time Period (T): The time taken for one complete oscillation.
    • Unit: seconds (s).
    • Relationship with frequency (f): T = 1/f or f = 1/T.

5. Properties of Sound:

  • Loudness:
    • Determined by the amplitude of the vibration.
    • Proportional to the square of the amplitude (Loudness ∝ Amplitude²).
    • Unit: decibel (dB).
    • Sounds above 80 dB can be physically painful and harmful.
  • Pitch:
    • Determined by the frequency of the vibration.
    • Higher frequency = Higher pitch (shriller sound, e.g., a whistle).
    • Lower frequency = Lower pitch (deeper/graver sound, e.g., a drum).
    • Example: A bird makes a high-pitched sound (high frequency), while a lion's roar is low-pitched (low frequency).

6. Audible and Inaudible Sounds:

  • Audible Range: The range of frequencies that the human ear can detect.
    • Typically 20 Hz to 20,000 Hz (or 20 kHz).
  • Inaudible Sounds: Sounds with frequencies outside the human audible range.
    • Infrasound: Frequencies below 20 Hz. Produced by sources like earthquakes, volcanoes, elephants, rhinos.
    • Ultrasound: Frequencies above 20,000 Hz (20 kHz). Produced and detected by animals like bats, dolphins, dogs.
  • Applications of Ultrasound: Medical imaging (ultrasonography), breaking kidney stones, cleaning intricate parts, detecting flaws in metal blocks, SONAR (Sound Navigation and Ranging).

7. Music and Noise:

  • Music: Sound that is pleasant to the ear. Often produced by regular, periodic vibrations. Examples: Sound from musical instruments played properly.
  • Noise: Sound that is unpleasant to the ear. Often produced by irregular, non-periodic vibrations. Examples: Sound from construction sites, traffic horns, classroom chatter.

8. Noise Pollution:

  • Definition: The presence of excessive or unwanted sound in the environment.
  • Sources: Road traffic, aircraft, industrial machines, construction activities, loudspeakers, bursting crackers, home appliances (mixers, vacuum cleaners, TVs at high volume).
  • Harmful Effects:
    • Health problems: Lack of sleep, hypertension (high blood pressure), anxiety, stress.
    • Temporary or permanent hearing impairment (especially due to prolonged exposure to loud noise).
  • Control Measures:
    • Installing silencing devices in vehicles, aircraft engines, industrial machines, and home appliances.
    • Running noisy operations away from residential areas.
    • Controlling the use of vehicle horns and loudspeakers.
    • Planting trees along roads and around buildings (trees absorb sound).
    • Setting limits on noise levels and timings for certain activities.
    • Using ear protection in noisy environments.

Multiple Choice Questions (MCQs):

  1. Sound is produced by:
    (a) Non-vibrating objects only
    (b) Objects in vacuum
    (c) Vibrating objects
    (d) Objects moving at high speed

  2. Sound cannot travel through:
    (a) Solids
    (b) Liquids
    (c) Gases
    (d) Vacuum

  3. The unit of frequency is:
    (a) Decibel (dB)
    (b) Meter (m)
    (c) Hertz (Hz)
    (d) Second (s)

  4. The loudness of a sound is primarily determined by its:
    (a) Frequency
    (b) Amplitude
    (c) Time period
    (d) Speed

  5. The pitch of a sound is primarily determined by its:
    (a) Frequency
    (b) Amplitude
    (c) Loudness
    (d) Medium

  6. The human ear can generally detect sound waves in the frequency range of:
    (a) 20 Hz to 2000 Hz
    (b) 2 Hz to 20 Hz
    (c) 20 Hz to 20,000 Hz
    (d) 2000 Hz to 200,000 Hz

  7. Sound travels fastest in which of the following media?
    (a) Air
    (b) Water
    (c) Steel
    (d) Vacuum

  8. The part of the human ear that vibrates first when sound waves enter it is the:
    (a) Cochlea
    (b) Auditory nerve
    (c) Eardrum
    (d) Middle ear bones

  9. Unpleasant sound is called:
    (a) Music
    (b) Noise
    (c) Frequency
    (d) Pitch

  10. Which of the following is NOT a major source of noise pollution?
    (a) Sound from a flute
    (b) Loudspeakers at high volume
    (c) Honking of vehicle horns
    (d) Machines in a factory

Answer Key for MCQs:

  1. (c) Vibrating objects
  2. (d) Vacuum
  3. (c) Hertz (Hz)
  4. (b) Amplitude
  5. (a) Frequency
  6. (c) 20 Hz to 20,000 Hz
  7. (c) Steel
  8. (c) Eardrum
  9. (b) Noise
  10. (a) Sound from a flute (generally considered music, not noise pollution unless extremely loud and unwanted)

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