NOTES Chapter 12: "Sound" from Class 9 Science CBSE NCERT:

 summary of Chapter 12: "Sound" from Class 9 Science CBSE NCERT:

1. What is Sound?

  • Sound is a form of energy that is produced by the vibration of objects and propagates through a medium (solid, liquid, or gas) in the form of waves.

  • Sound travels in waves and can be heard when it reaches our ears.

  • How is sound produced?

    • Sound is produced when an object vibrates. These vibrations cause the particles of the medium to move, creating compressions and rarefactions (regions of high and low pressure) that travel through the medium.

2. Propagation of Sound

  • Sound needs a medium (like air, water, or solid objects) to travel. It cannot travel in a vacuum.

  • How sound waves travel:

    • Sound waves are longitudinal waves (particles of the medium move in the same direction as the wave).

    • When an object vibrates, it creates a series of compressions (regions where particles are close together) and rarefactions (regions where particles are spread apart) in the surrounding medium. These compressions and rarefactions move as waves.

    • Example: When you pluck a guitar string, the vibrations create sound waves that travel through the air to your ears.

3. Characteristics of Sound Waves

The properties of sound waves determine how we perceive them. These include:

  • Frequency (f):

    • The number of vibrations (or cycles) per second.
    • It determines the pitch of sound.
    • High frequency = high pitch (like a whistle), and low frequency = low pitch (like a drum).

  • Amplitude (A):

    • The maximum displacement of particles from their rest position during vibration.
    • It determines the loudness of the sound.
    • Higher amplitude = louder sound, and lower amplitude = softer sound.

  • Wavelength (λ):

    • The distance between two consecutive compressions or rarefactions.
    • It is inversely related to the frequency: Higher frequency = shorter wavelength.

  • Speed of Sound:

    • The speed at which sound travels depends on the medium. It travels faster in solids than in liquids, and faster in liquids than in gases.
    • In air, the speed of sound is approximately 343 m/s at room temperature.

4. Speed of Sound in Different Media

  • In Air: The speed of sound is about 343 m/s at 20°C.
  • In Water: Sound travels faster, around 1482 m/s.
  • In Solids: Sound travels even faster, for example, in steel, it travels at about 5000 m/s.
  • The speed of sound increases with the increase in temperature (because molecules move faster).

5. Reflection of Sound

  • Reflection of sound occurs when sound waves bounce off a surface.
  • We use the principle of reflection of sound in various applications:
    • Echo: A reflected sound heard after a time delay.
    • Example: You shout in an empty hall or a valley, and after a while, you hear your voice again — that is the echo.
  • Law of Reflection:
    • The angle of incidence is equal to the angle of reflection. This applies to sound waves, just like light.

6. Application of Reflection of Sound

  • Echo:
    • The repetition of sound caused by the reflection of sound waves. It can be heard clearly when the time difference between the original sound and the reflected sound is more than 0.1 seconds.
    • Echoes are used in sonar technology to detect objects underwater.
  • Sonar (Sound Navigation and Ranging):
    • It is used by ships and submarines to detect objects underwater.
    • Sonar emits sound waves, and by measuring the time it takes for the sound to reflect back, the distance to the object can be determined.

7. Refraction of Sound

  • Refraction occurs when sound waves change speed as they move through different mediums, leading to a change in direction.
  • This happens because sound travels at different speeds in different media. For example, sound travels faster in warm air and slower in cold air, which can cause sound waves to bend.

8. Doppler Effect

  • The Doppler Effect is the change in frequency or pitch of sound due to the motion of the source or the observer.

  • Example:

    • When an ambulance with its siren passes by, the pitch of the sound appears to be higher as it approaches and lower as it moves away. This is because the sound waves are compressed as the ambulance approaches (higher frequency), and stretched as it moves away (lower frequency).

  • The Doppler Effect is used in radar and sonar to measure the speed of objects, including vehicles, rainstorms, and even stars.

9. Sound and its Applications

  • Musical Instruments: Different musical instruments produce sound by vibrating strings (guitar, violin), air columns (flute, trumpet), or membranes (drums).

  • Ultrasound:

    • Ultrasound refers to sound waves with frequencies higher than the human audible range (>20,000 Hz). These are used in medical imaging (e.g., sonography) to create images of internal organs and tissues.

  • Infrasound:

    • Infrasound refers to sound waves with frequencies lower than the human hearing range (<20 Hz). These are used in earthquake detection and studying animal behavior (like elephants communicating).

10. Hearing Range of Humans

  • Human Hearing Range: The typical human ear can hear sound frequencies between 20 Hz to 20,000 Hz.
  • Sounds below 20 Hz are called infrasound, and those above 20,000 Hz are called ultrasound.

11. Noise and Sound Pollution

  • Noise is unwanted or harmful sound. It can affect human health, causing stress, hearing loss, sleep disturbances, and even heart disease.

  • Sound Pollution (or Noise Pollution) refers to harmful levels of sound in the environment. It can be caused by vehicles, construction, loudspeakers, etc.

  • Ways to Control Noise Pollution:

    • Using quieter machines
    • Soundproofing buildings
    • Using ear protection in noisy environments

Key Takeaways

  1. Sound is produced by vibrating objects and needs a medium to travel through.
  2. Sound waves are longitudinal waves and consist of compressions and rarefactions.
  3. Frequency determines pitch, and amplitude determines loudness.
  4. Sound travels faster in solids than in liquids and faster in liquids than in gases.
  5. Reflection of sound causes echoes, and this principle is used in sonar.
  6. The Doppler Effect explains how the pitch of sound changes with motion.
  7. Noise pollution can be harmful and can be controlled with proper measures.

Important Formulas

  1. Speed of Sound: Depends on the medium and temperature.
  2. Wavelength, Frequency, and Speed Relationship: v=f×λv = f \times \lambdaWhere:
    • vv = Speed of sound
    • ff = Frequency
    • λ\lambda = Wavelength

Comments

Post a Comment

Popular posts from this blog

NOTES Chapter 15: Our Environment Class 10 Science – CBSE NCERT

  Chapter 15: Our Environment Class 10 Science – CBSE NCERT Chapter 15 focuses on the various components that make up our environment, including ecosystems , biotic and abiotic factors , and food chains and webs . It highlights the balance of nature , biodiversity , and environmental issues like pollution, deforestation, and the need for sustainable development. The chapter also explains the impact of human activities on the environment and emphasizes the importance of conserving natural resources for future generations. 1. The Environment and Its Components The environment is the surrounding in which organisms live. It includes both living and non-living things and can be broadly classified into two categories: 1.1 Biotic Components : These are the living organisms in the environment. They include: Plants (producers), Animals (consumers), Microorganisms (decomposers). 1.2 Abiotic Components : These are the non-living factors that affect the living organisms in th...
Read more

Moments Chapter 3 of CBSE Class 9, titled "Iswaran the Storyteller" by R.K. Narayan:

  summary of Chapter 3 of Moments for CBSE Class 9, titled "Iswaran the Storyteller" by R.K. Narayan : Summary: In this chapter, the story is centered around Iswaran , a cook who works for the narrator, and his fascinating way of telling stories. Iswaran's storytelling skills and his adventurous nature provide both entertainment and a sense of mystery to the narrator, who works in a small town. The narrator begins by describing Iswaran as an enthusiastic and talented cook who makes delicious food and is always happy to serve. Iswaran is very devoted to his job and never seems to get tired, no matter how much work is piled up. He is also well known for his remarkable storytelling abilities. He often entertains the narrator with thrilling stories, which he describes in vivid detail. The Background of Iswaran’s Life: Iswaran is originally from a village, and he later moves to the town where he works for the narrator. He is described as a "man of strong character...
Read more