Sound Class 9 Science — Revision Notes
This chapter explores properties and propagation of sound waves. Students learn about frequency, wavelength, speed of sound, and practical applications.
TL;DR: This chapter explores properties and propagation of sound waves. Students learn about frequency, wavelength, speed of sound, and practical application…
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This chapter explores properties and propagation of sound waves. Students learn about frequency, wavelength, speed of sound, and practical applications.
Sound Waves
- Sound is mechanical wave requiring medium for propagation
- Sound cannot travel in vacuum
- Sound waves are longitudinal waves with compressions and rarefactions
- Requires vibrating source to produce sound
Properties of Sound
- Frequency: number of vibrations per second (unit: hertz, Hz)
- Wavelength: distance between consecutive compressions or rarefactions
- Wave speed: v = f x λ where f is frequency, λ is wavelength
- Amplitude: maximum displacement from mean position
Speed of Sound
- Speed in air at 0°C: 331 m/s
- Speed in water: 1450 m/s (faster than air)
- Speed in solid: 5000 m/s (faster than water)
- Speed increases with temperature
Doppler Effect
- Apparent frequency changes when source or observer is moving
- Frequency increases when source approaches observer
- Frequency decreases when source moves away
- Observed frequency: f = f0 (v ± vo) / (v ± vs) where v is sound speed
Key Terms
- Frequency: Number of vibrations per second (measured in hertz)
- Wavelength: Distance between consecutive compressions or rarefactions
- Doppler Effect: Change in apparent frequency due to relative motion of source and observer
Frequently Asked Questions
Why cannot sound travel in vacuum?
Sound is mechanical wave requiring medium (particles) to propagate. In vacuum, there are no particles to vibrate and transmit sound waves.
What is relationship between frequency and wavelength?
v = f x λ. For constant wave speed, frequency and wavelength are inversely proportional. Higher frequency means shorter wavelength.
More Class 9 Science Revision Notes
- Matter in Our Surroundings
- Is Matter Around Us Pure
- Atoms and Molecules
- Structure of the Atom
- The Fundamental Unit of Life
- Tissues
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