Chapter 10: Sound Waves: Characteristics and Applications
Original notes — ready CBSE (NCERT, English Medium) • Class 9 • Science • Book: Exploration
Chapter 10: Sound Waves: Characteristics and Applications — EG's own notes, in simple words
Book: Exploration (CBSE, Class 9) | Chapter 10 | Medium: English
Diagram (labels are printed on the image): Diagram (labels are printed on the image): Sound from a vibrating source travels as waves through a medium such as air. A general diagram for study, not a figure copied from the book.
Sound around us — music, speech and echoes. A general picture about sound, not a book scene.
What the chapter is about
Sound is produced by vibrating objects. When a drum is struck, its skin vibrates; when we speak, our vocal cords vibrate. These vibrations disturb the air around them, and the disturbance travels onwards as a wave until it reaches our ears.
Sound needs a medium to travel — it can pass through solids, liquids and gases, but not through a vacuum. A sound wave can be described by its frequency, the number of vibrations per second, measured in hertz; its amplitude, which decides loudness; and its wavelength, the distance between two matching points of the wave. Higher frequency means a higher pitch, and larger amplitude means a louder sound.
Sound also reflects. The echo we hear in an empty hall is reflected sound, and this reflection is put to work in SONAR, which finds objects under water, and in ultrasound scans, which look inside the body without harm. Sounds above the human hearing range are called ultrasound, and sounds below it are called infrasound.
What you should learn
Notice that sound is vibration travelling through a medium, and how frequency, amplitude and wavelength describe every sound wave.
Key Terms
- Vibration — Rapid back-and-forth movement of an object, which produces sound.
- Frequency — The number of vibrations per second; measured in hertz (Hz).
- Amplitude — The size of a vibration; it decides how loud a sound is.
- Wavelength — The distance between two matching points of a wave, such as two crests.
- Echo — Sound heard again after reflection from a surface.
Remember
- Sound is produced by vibration and needs a medium to travel.
- Sound cannot travel through a vacuum.
- Higher frequency = higher pitch; larger amplitude = louder sound.
- Echo is reflected sound; SONAR uses it under water.
Exam Tip / Common Mistake
Common mistake: saying sound travels fastest in air. Sound actually travels fastest in solids, then liquids, then gases, because the particles are closer together and pass the vibration on more quickly.
Chart / Table: Sound waves at a glance
| Point | Detail |
|---|---|
| Frequency | Vibrations per second; unit hertz (Hz); decides pitch |
| Amplitude | Size of vibration; decides loudness |
| Wavelength | Distance between two crests |
| Speed in air | About 344 metres per second |
| Ultrasound | Sound above the human hearing range |
Practice set — try it yourself
- Why can astronauts not hear each other in space without radios?
Hint: Think about what sound needs to travel. - What decides the pitch of a sound?
Hint: Think about vibrations per second. - Why does a drum sound louder when struck harder?
Hint: Think about the size of vibration. - What is an echo? How is it used in SONAR?
Hint: Think about reflection. - Give one medical use of ultrasound.
Hint: Think about looking inside the body safely.
Questions with Answers
- How is sound produced?
Sound is produced by a vibrating object. The vibration disturbs the particles of the medium around it, and this disturbance travels as a wave to our ears. - Why does sound need a medium?
Sound travels as vibrations of particles. In a vacuum there are no particles to vibrate, so sound cannot travel through it. - What is the difference between frequency and amplitude?
Frequency is the number of vibrations per second, measured in hertz, and it decides pitch. Amplitude is the size of the vibration, and it decides loudness. - What is an echo, and how is it useful?
An echo is sound heard again after reflection from a surface. SONAR sends sound waves into water and measures the time of the returning echo to find the distance and position of objects. - What is ultrasound? Name one use.
Ultrasound is sound with a frequency above the human hearing range (above 20,000 Hz). It is used in medical scans to look inside the body, and in industry to clean parts and detect cracks.
How to use this page: Read the summary, say the Key Terms in your own words, then try the practice set without looking. Use the Questions with Answers to check yourself afterwards. Ask your teacher or parent to check your answers.