Chapter – 11

Sound

In this post we have given the detailed notes of class 9 Science Chapter 11 (Sound) in English. These notes are useful for the students who are going to appear in class 9 board exams.

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Chapter 11: Sound

• Sound is a form of energy that we hear from various sources like humans, birds, bells, machines, etc.  
• Sound is produced by vibrating objects. 📳 When an object vibrates, it causes the air particles around it to vibrate as well, creating sound waves. 〰️
• Examples:
  • When you clap your hands, the sound is produced due to the vibration of your hands. 👏
  • When a guitar string is plucked, the vibration of the string produces sound. 🎸
  • When a drum is hit, the vibration of the drumhead produces sound. 🥁

11.2 Propagation of Sound 📢

• Sound needs a medium to travel. It can be a solid, liquid, or gas. 🪨💧💨
• Sound travels through a medium by compressions (high-pressure regions) and rarefactions (low-pressure regions). 🔽🔼
• Sound waves are longitudinal waves because the particles of the medium vibrate in the same direction as the wave travels. ➡️➡️
• Characteristics of a sound wave:
  • Wavelength (λ): The distance between two consecutive compressions or rarefactions. 📏
  • Frequency (ν): The number of oscillations per unit time. ⏱️
  • Time period (T): The time taken for one complete oscillation. 🔄
  • Amplitude (A): The magnitude of maximum disturbance in the medium. 🔼

11.3 Reflection of Sound 🪞

• Sound waves can reflect off surfaces, just like light waves. 💡
• The laws of reflection are the same for sound as for light. 📐
• Echo: A reflected sound wave that is heard separately from the original sound. 🗣️⛰️
• Reverberation: The persistence of sound in a space due to multiple reflections. 🏟️
• Applications of multiple reflection of sound:
  • Megaphones 📢
  • Horns 📯
  • Stethoscopes 🩺
  • Concert halls 🎶

11.4 Range of Hearing 👂

• Audible range: The range of frequencies that humans can hear (20 Hz to 20,000 Hz).  
• Infrasound: Sound waves with frequencies below 20 Hz. 🐘
• Ultrasound: Sound waves with frequencies above 20,000 Hz. 🦇

11.5 Applications of Ultrasound 🏥🏭

• Industrial applications:
  • Cleaning 🧼
  • Detecting cracks and flaws in metals 🛠️
• Medical applications:
  • Ultrasonography: Imaging internal organs  
  • Breaking kidney stones 🤕

Key Points to Remember 📌

• Sound is produced by vibrations. 📳
• Sound needs a medium to travel. 📢
• Sound waves are longitudinal waves. ➡️➡️
• Sound can be reflected. 🪞
• Humans can hear sounds in the audible range (20 Hz to 20,000 Hz).  
• Ultrasound has many applications in industry and medicine. 🏥🏭

We hope that class 9 Science Chapter 11 (Sound) notes in English helped you. If you have any query about class 9 Science Chapter 11 (Sound) notes in English or about any other notes of class 9 Science in English, so you can comment below. We will reach you as soon as possible…