Class 8 Science Notes Chapter 13 (Chapter 13) – Examplar Problems Book

Alright class, let's focus on Chapter 13, 'Sound', from your Science Exemplar book. This is an important topic, not just for your class but also for various government exams where basic science concepts are often tested. Pay close attention to the details.

Chapter 13: Sound - Detailed Notes for Exam Preparation

1. What is Sound and How is it Produced?

• Definition: Sound is a form of energy that travels as waves and causes the sensation of hearing when it reaches our ears.
• Production: Sound is produced by vibrating objects. Vibration means a rapid to-and-fro or back-and-forth motion of an object.
  • Examples:
    • Plucking a stretched rubber band makes it vibrate and produce sound.
    • Striking a drum makes its membrane vibrate.
    • Our own voice is produced by the vibration of vocal cords in the larynx (voice box). The tightness and thickness of vocal cords determine the type of voice.

2. Propagation of Sound (How Sound Travels)

• Medium Required: Sound needs a material medium (solid, liquid, or gas) to travel from the source to the listener. Vibrations pass from one particle of the medium to the next.
• No Propagation in Vacuum: Sound cannot travel through a vacuum (a space with no matter/air). This is proven by the bell jar experiment where a ringing bell inside an evacuated jar cannot be heard.
• Speed of Sound: Sound travels at different speeds in different media. It travels:
  • Fastest in solids.
  • Slower in liquids.
  • Slowest in gases (Speed of sound in air is approx. 343 m/s at 20°C).

3. The Human Ear - How We Hear

• The ear is our sense organ for hearing.
• Outer Ear (Pinna): Collects sound waves.
• Ear Canal: Sound waves travel through this tube.
• Eardrum (Tympanic Membrane): A thin, stretched membrane at the end of the ear canal. It vibrates when sound waves strike it.
• Middle Ear: Contains three tiny, interconnected bones (hammer, anvil, stirrup) that amplify the vibrations received from the eardrum.
• Inner Ear (Cochlea): A spiral-shaped organ filled with fluid. It converts the amplified pressure variations (vibrations) into electrical signals.
• Auditory Nerve: Transmits these electrical signals to the brain.
• Brain: Interprets these signals as sound.

4. Characteristics of Sound Waves

• Sound travels as waves, characterized by oscillations.
• Oscillation: One complete to-and-fro movement of a vibrating particle.
• Amplitude (A):
  • The maximum displacement or distance moved by a point on a vibrating body or wave measured from its equilibrium position.
  • Determines the loudness of the sound.
  • Larger amplitude = Louder sound.
  • Smaller amplitude = Softer sound.
  • Loudness is measured in decibels (dB). Sounds above 80 dB can be physically painful and harmful.
• Frequency (f):
  • The number of oscillations (vibrations) per second.
  • Determines the pitch (shrillness) of the sound.
  • Higher frequency = Higher pitch (shriller sound, e.g., a bird's chirp).
  • Lower frequency = Lower pitch (flatter or deeper sound, e.g., a lion's roar).
  • Unit: Hertz (Hz). (1 Hz = 1 vibration per second).
• Time Period (T):
  • The time taken to complete one oscillation.
  • Unit: seconds (s).
  • Relationship: Frequency and Time Period are inversely related: f = 1/T or T = 1/f.

5. Audible and Inaudible Sounds

• Audible Range: The range of sound frequencies that the human ear can detect. It is typically from 20 Hz to 20,000 Hz (or 20 kHz).
• Inaudible Sounds: Sounds outside this frequency range cannot be heard by humans.
  • Infrasound: Frequencies below 20 Hz. (e.g., produced by earthquakes, rhinoceroses, whales).
  • Ultrasound: Frequencies above 20,000 Hz (20 kHz). (e.g., used by bats and dolphins for echolocation, in medical sonography, and for industrial purposes).

6. Music vs. Noise

• Music: Sound that is pleasant, ordered, and generally produced by regular, periodic vibrations. (e.g., sound from musical instruments played properly).
• Noise: Sound that is unpleasant, unwanted, and generally produced by irregular, non-periodic vibrations. (e.g., sound of traffic, construction work, loud arguments).

7. Noise Pollution

• Definition: The presence of excessive, unwanted, and disturbing sound in the environment.
• Major Sources:
  • Road traffic (vehicles, horns)
  • Air traffic (airplanes)
  • Construction sites (machinery)
  • Factories (industrial machines)
  • Loudspeakers, high volume music systems
  • Household appliances (mixers, vacuum cleaners)
  • Bursting of firecrackers
• Harmful Effects: Noise pollution can cause various health problems:
  • Hearing loss (temporary or permanent)
  • Stress and anxiety
  • Hypertension (high blood pressure)
  • Sleep disturbances (insomnia)
  • Lack of concentration
• Measures to Control Noise Pollution:
  • Source Reduction: Using silencers in vehicles and machines, designing quieter engines, restricting horn usage, banning loud firecrackers.
  • Path Control: Planting trees and shrubs along roads and around buildings (they act as sound absorbers), building noise barriers.
  • Receiver Protection: Using earplugs or earmuffs in noisy environments.
  • Legislation & Awareness: Enforcing noise limits, creating 'silence zones' near hospitals and schools, educating the public about the harms of noise pollution.

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on Chapter 13 - Sound:

1. Sound is produced due to:
   (a) Friction between objects
   (b) Motion of objects
   (c) Vibration of objects
   (d) Refraction of light

2. Through which of the following media can sound NOT travel?
   (a) Water
   (b) Steel
   (c) Air
   (d) Vacuum

3. The characteristic of sound that determines its pitch is:
   (a) Amplitude
   (b) Frequency
   (c) Speed
   (d) Loudness

4. The unit used to measure the loudness of sound is:
   (a) Hertz (Hz)
   (b) Meter per second (m/s)
   (c) Second (s)
   (d) Decibel (dB)

5. The range of audible frequencies for the human ear is approximately:
   (a) 2 Hz to 20 Hz
   (b) 20 Hz to 20,000 Hz
   (c) 200 Hz to 20,000 Hz
   (d) Above 20,000 Hz

6. Sound waves with frequencies below 20 Hz are called:
   (a) Ultrasound
   (b) Supersonic sound
   (c) Infrasound
   (d) Audible sound

7. Which part of the human ear converts sound vibrations into electrical signals?
   (a) Eardrum
   (b) Hammer, Anvil, Stirrup
   (c) Cochlea
   (d) Auditory nerve

8. A sound is louder if its:
   (a) Frequency is higher
   (b) Amplitude is larger
   (c) Speed is greater
   (d) Time period is shorter

9. Which of the following is NOT an effective measure to control noise pollution?
   (a) Planting trees along roadsides
   (b) Using silencers in vehicles
   (c) Playing loud music regularly
   (d) Restricting the use of loudspeakers

10. Bats use which type of sound waves for navigation and hunting?
    (a) Infrasound
    (b) Audible sound
    (c) Ultrasound
    (d) Musical sound

Answer Key for MCQs:

1. (c)
2. (d)
3. (b)
4. (d)
5. (b)
6. (c)
7. (c)
8. (b)
9. (c)
10. (c)

Make sure you understand these concepts thoroughly. Revise them regularly for your exams. Let me know if any part needs further clarification.