"Sound" chapter:10 of Class 8 Science (CBSE):-

Extra 20 important long questions with answers

1. What is sound, and how is it produced?

✅ Answer:

Sound is a form of energy produced by vibrating objects.

The to-and-fro motion of an object is called vibration.

When an object vibrates, it creates disturbances in the medium around it.

These disturbances travel as sound waves.

Sound requires a medium (solid, liquid, or gas) to travel.

Examples: Guitar strings, vocal cords, tuning fork.

 

2. How does sound propagate through a medium?

✅ Answer:

Sound propagates in the form of longitudinal waves.

These waves consist of compressions and rarefactions.

Compressions are regions where particles are close together.

Rarefactions are regions where particles are far apart.

The energy is transferred from one particle to another.

It cannot travel in a vacuum as there are no particles to carry the waves.

 

3. What are the characteristics of sound? Explain each.

✅ Answer:

Loudness: Depends on the amplitude of vibrations; measured in decibels (dB).

Pitch: Determines how high or low a sound is; depends on frequency.

Quality (Timbre): Distinguishes sounds of the same loudness and pitch but from different sources.

Speed of Sound: Depends on the medium (faster in solids, slower in gases).

Echo: Reflection of sound when it bounces off a surface.

Persistence of Sound: Sound lingers briefly after the source stops vibrating.

 

4. Explain the different types of waves in which sound travels.

✅ Answer:

Sound travels as mechanical waves that require a medium.

It moves in the form of longitudinal waves in which particles move parallel to wave direction.

Compressions (high-pressure regions) and rarefactions (low-pressure regions) are formed.

In solids, sound can also travel as transverse waves under specific conditions.

Sound waves travel at different speeds in different media.

They cannot travel through a vacuum due to the absence of a medium.

 

5. What are the factors affecting the speed of sound?

✅ Answer:

Medium: Sound travels fastest in solids, slower in liquids, and slowest in gases.

Density: Higher density means slower sound travel (except in solids).

Temperature: Higher temperature increases sound speed (e.g., faster in warm air).

Humidity: More humidity increases sound speed in the air.

Pressure: In gases, under constant temperature, pressure does not affect sound speed.

Elasticity: More elastic substances allow faster sound propagation.

 

6. Explain the concept of frequency and its relation to pitch.

✅ Answer:

Frequency is the number of vibrations per second (unit: Hertz).

Higher frequency = higher pitch (e.g., a whistle).

Lower frequency = lower pitch (e.g., a drum).

Human ears can hear frequencies between 20 Hz to 20,000 Hz.

Sounds above 20,000 Hz are ultrasonic, and below 20 Hz are infrasonic.

Example: A woman’s voice has a higher pitch than a man's due to higher frequency.

 

7. What is an echo? State the conditions necessary to hear an echo.

✅ Answer:

An echo is the reflection of sound from a hard surface.

The minimum distance for an echo to be heard is 17.2 meters.

The surface must be hard and large, like a mountain or a wall.

The time interval between the original and reflected sound must be at least 0.1 seconds.

Echoes are used in SONAR and medical imaging.

Soft surfaces like curtains and carpets absorb sound, reducing echoes.

 

8. What is noise pollution? List its causes and effects.

✅ Answer:

Noise pollution is excessive and unwanted sound in the environment.

Causes:

Traffic noise (vehicles, horns).

Loudspeakers and industrial machines.

Construction activities.

Household appliances like mixers and TVs at high volume.

Effects:

Hearing loss.

Increased stress and lack of concentration.

Sleep disturbances.

Increased heart-related issues.

 

9. How can noise pollution be controlled?

✅ Answer:

Use soundproofing materials in buildings.

Ban loudspeakers in residential areas after specific hours.

Promote planting trees to absorb sound.

Enforce traffic regulations to minimize unnecessary honking.

Maintain industrial machines to reduce noise production.

Raise awareness about the harmful effects of noise pollution.

 

10. Explain the human ear’s structure and function in hearing sound.

✅ Answer:

The human ear detects and processes sound vibrations.

It consists of three parts:

Outer ear: Collects sound and directs it to the eardrum.

Middle ear: Amplifies sound using three tiny bones (hammer, anvil, stirrup).

Inner ear: Converts vibrations into electrical signals for the brain.

The eardrum vibrates when sound waves hit it.

Vibrations are transmitted to the inner ear via ossicles.

The cochlea converts them into nerve impulses.

The brain interprets these impulses as sound.

 

11. What is SONAR? Mention its applications.

✅ Answer:

SONAR stands for Sound Navigation and Ranging.

It uses ultrasonic waves to detect objects underwater.

The waves are sent out, and their reflected waves are recorded.

The time taken for reflection determines the object’s distance.

Applications:

Used in submarines to locate enemy ships.

Helps fishermen locate fish in deep waters.

Used in ocean mapping and shipwreck detection.

 

12. What is the difference between music and noise?

✅ Answer:

Music	Noise
Pleasant and rhythmic sound.	Unpleasant, irregular sound.
Follows a pattern.	Random and chaotic vibrations.
Created by musical instruments.	Created by honking, shouting, machines.
Soothing to ears.	Can cause discomfort or harm.
Example: Songs, instruments.	Example: Traffic, loudspeakers.

 

 

13. What are the differences between transverse and longitudinal waves?

✅ Answer:

Feature	Transverse Waves	Longitudinal Waves
Direction of vibration	Perpendicular to wave motion	Parallel to wave motion
Medium required	Can travel in solids & liquids	Require a medium (solid, liquid, gas)
Example	Water waves, light waves	Sound waves, seismic P-waves
Regions in waves	Crests & troughs	Compressions & rarefactions
Sound type	Not applicable for sound	Sound always travels as longitudinal waves

 

14. What are ultrasonic waves? Mention their uses.

✅ Answer:

Ultrasonic waves are sound waves with frequencies above 20,000 Hz.

They are inaudible to humans but can be heard by some animals (e.g., bats, dolphins).

Uses of ultrasonic waves:

SONAR for detecting underwater objects.

Medical imaging (Ultrasound scans) to examine internal body organs.

Cleaning delicate objects (e.g., jewelry, watches) using ultrasonic cleaners.

Detecting cracks in machinery using ultrasound.

Bats and dolphins use them for navigation and hunting.

 

15. Why does sound travel faster in solids than in gases?

✅ Answer:

The speed of sound depends on how closely packed the molecules are.

In solids, molecules are tightly packed, so vibrations pass quickly.

In liquids, molecules are less packed, so sound is slower than in solids.

In gases, molecules are far apart, making sound travel slowest.

Sound waves rely on particle collisions to propagate.

Example: Sound travels fastest in steel and slowest in air.

 

16. What are infrasonic waves? Give examples of their sources.

✅ Answer:

Infrasonic waves are sound waves with frequencies below 20 Hz.

Humans cannot hear infrasonic sounds.

Some animals, like elephants and whales, use them for communication.

Sources of infrasonic waves:

Earthquakes and volcanic eruptions.

Ocean waves and thunderstorms.

Large vibrating machines.

Animals like giraffes, elephants, and rhinoceroses.

These waves can travel long distances without losing energy.

Scientists use infrasonic waves to detect earthquakes and nuclear explosions.

 

17. What is amplitude? How is it related to loudness?

✅ Answer:

Amplitude is the maximum displacement of particles in a sound wave.

It represents the energy of the wave.

Louder sound → Higher amplitude; softer sound → Lower amplitude.

Loudness is measured in decibels (dB).

Examples:

A whisper (~30 dB) has low amplitude.

A rock concert (~110 dB) has high amplitude.

Sound can damage ears if the amplitude is too high (above 85 dB).

 

18. Why do we hear the sound of a thunderstorm after seeing the lightning?

✅ Answer:

Light and sound travel at different speeds.

The speed of light in air = 300,000 km/s (very fast).

The speed of sound in air = 343 m/s (much slower).

During a thunderstorm, lightning and thunder occur at the same time.

But we see lightning first because light reaches us faster.

We hear thunder later as sound takes more time to travel.

 

19. What is reverberation? How can it be reduced?

✅ Answer:

Reverberation is the prolonged persistence of sound due to multiple reflections.

It happens in large empty halls or auditoriums.

The sound waves bounce off walls, ceilings, and floors, making it unclear.

Ways to reduce reverberation:

Using curtains and carpets to absorb sound.

Installing acoustic panels in auditoriums.

Covering walls with foam or wooden materials.

Reducing hard surfaces that reflect sound.

Using sound-absorbing ceiling tiles.

 

20. Why do different musical instruments produce different sounds?

✅ Answer:

Musical instruments produce sound by vibrating strings, air, or surfaces.

Each instrument has a unique shape and material, affecting the sound.

The length, thickness, and tension of strings change pitch and quality.

Wind instruments (flute, trumpet) depend on air column vibrations.

Percussion instruments (drums, tabla) depend on membrane vibrations.

This difference in vibration patterns and overtones creates unique sounds.