Class 9 Science Exploration Solutions English Medium

NCERT Solutions Class 9 Science Exploration English Medium

Sound Waves: Characteristics and Applications

Topic: Speed of Sound

Chapter 10. Sound Waves: Characteristics and Applications

This section explains the speed of sound, the factors affecting it and how sound travels at different speeds in solids, liquids and gases. It also includes practical applications and solved numerical problems based on the speed of sound.

Speed of Sound

The speed of sound is the distance travelled by a sound wave in one second. It depends on the nature of the medium and environmental conditions.

Definition

The speed of sound is the distance travelled by a sound wave in unit time.

Formula

v = νλ

Where:

• v = Speed of sound (m/s)
• ν = Frequency (Hz)
• λ = Wavelength (m)

Speed of Sound in Different Media

Sound travels at different speeds depending on the medium through which it propagates.

Factors Affecting the Speed of Sound

• Nature of the Medium – Sound travels faster in solids than in liquids and gases.
• Temperature – The speed of sound increases with an increase in temperature.
• Density and Elasticity – The speed depends on the elastic properties and density of the medium.

Effect of Temperature

• Warm air allows sound to travel faster than cold air.
• As temperature increases, the particles move more rapidly.
• Faster particle motion helps transfer sound energy more quickly.

Examples from Daily Life

• Railway tracks help us hear an approaching train earlier because sound travels faster through steel than through air.
• Whales and dolphins communicate effectively in water because sound travels faster in water than in air.
• Thunder is heard after lightning because sound travels much slower than light.

Solved Numerical – 1

Question: A sound wave has a frequency of 400 Hz and a wavelength of 0.85 m. Calculate its speed.

Solution:

Given:

• Frequency (ν) = 400 Hz
• Wavelength (λ) = 0.85 m

v = νλ

v = 400 × 0.85 = 340 m/s

Answer: Speed of sound = 340 m/s.

Solved Numerical – 2

Question: The speed of sound is 330 m/s and its frequency is 660 Hz. Find the wavelength.

Solution:

Using the formula:

λ = v / ν

λ = 330 / 660 = 0.5 m

Answer: Wavelength = 0.5 m.

Applications of the Speed of Sound

• Estimating the distance of thunderstorms.
• Communication in underwater environments.
• Designing auditoriums and concert halls.
• Medical imaging using ultrasonic waves.
• Navigation using SONAR technology.

ATP Education Concept Builder

The speed of sound depends on the medium, not on the loudness of the sound. In the same medium, changing the frequency changes the wavelength so that the speed remains constant.

ATP Education Exam Booster

• Memorise the formula v = νλ.
• Remember the order of speed: Solids > Liquids > Gases.
• Understand the effect of temperature on the speed of sound.
• Practise numerical problems based on speed, wavelength and frequency.
• CBSE competency-based questions often ask students to explain why sound reaches us faster through solids than through air.

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