Class 8 Chapter 10 Long Qa Science Cbse Ncert

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Extra 20 important long questions with answers from the "Sound" chapter:10 of Class 8 Science (CBSE):-

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.

solids

, molecules are tightly packed, so vibrations pass quickly.

liquids

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

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.

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

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.