Class 8 Science Chapter 13 – Sound | Full NCERT Notes
Introduction: The World of Sound
Imagine a world without sound — no music, no laughter, no chirping birds, and no spoken words. Life would feel strangely empty and disconnected. Sound is one of the most beautiful and vital sensations in our lives. It allows us to communicate, express emotions, and sense the world around us.
In science, sound is a form of energy produced by vibrating objects. It travels through different materials and reaches our ears, enabling us to hear. The study of sound helps us understand how musical instruments work, how doctors use ultrasound, and how animals communicate.
Sound surrounds us — from the honking of cars to the rustling of leaves, from a whisper to a thunderclap. Let’s explore how it’s produced, how it travels, and how we hear it.
How Sound Is Produced
Sound is always produced by vibration — a rapid back-and-forth motion of an object. Whenever something vibrates, it sets the nearby air particles in motion. These particles bump into their neighbors, passing the energy along until it reaches our ears.
Examples of Vibrating Objects
-
When you pluck a guitar string, it moves back and forth, creating vibrations that produce sound.
-
When you blow air into a flute, the column of air inside vibrates.
-
When you hit a drum, the stretched membrane vibrates, creating sound waves in air.
You can test it yourself:
Place your fingers lightly on your throat and speak. You will feel vibrations — those are your vocal cords moving!
Vibrations and Their Characteristics
Every sound-producing object vibrates in a particular pattern. The nature of these vibrations determines the pitch, loudness, and quality of the sound.
If you observe a tuning fork, its prongs move quickly back and forth when struck. This back-and-forth motion disturbs the air around it, producing sound waves.
When the vibrations stop, the sound stops too.
Propagation of Sound
Sound travels through a medium — such as air, water, or solids — by disturbing the particles of that medium. Each particle pushes or pulls its neighbor, transferring energy along a wave.
Sound cannot travel through a vacuum because there are no particles to carry the vibration.
Demonstration:
Take an electric bell in a glass jar and start it ringing. You can hear it clearly. Now remove the air with a vacuum pump — the sound fades until it’s nearly gone. This shows that air is necessary for sound to travel.
Sound Travels as a Wave
Sound travels as a mechanical wave, which means it needs a medium to propagate. It moves in the form of longitudinal waves — where particles of the medium vibrate back and forth in the same direction as the wave moves.
As the wave travels, it creates regions of compression (where particles are close together) and rarefaction (where they are far apart). These compressions and rarefactions alternate continuously.
Example:
When a tuning fork vibrates:
-
The prong moving forward compresses the air — creating compression.
-
The prong moving backward creates rarefaction.
This sequence repeats rapidly, forming a sound wave.
Sound Requires a Medium
Sound cannot travel through empty space. It can pass through:
-
Solids: Best medium (sound travels fastest)
-
Liquids: Medium speed
-
Gases: Slowest speed
For example, sound travels faster in steel than in water, and faster in water than in air.
That’s why railway workers place their ears on tracks to hear approaching trains from far away — vibrations travel faster through metal rails than through air.
Sound Waves and Their Features
Sound waves can be described using a few scientific terms:
Amplitude
The amplitude of a sound wave is the maximum distance particles move from their resting position. It determines the loudness of the sound. Greater amplitude means louder sound.
Frequency
The frequency is the number of vibrations (or cycles) per second. It determines the pitch (how high or low a sound is). Frequency is measured in Hertz (Hz).
-
High frequency = high-pitched sound (like a whistle).
-
Low frequency = low-pitched sound (like a drum).
Time Period
The time period is the time taken for one complete vibration. It is the inverse of frequency:
Wavelength
The wavelength is the distance between two consecutive compressions or rarefactions in a sound wave.
Amplitude, Frequency, and Loudness Relationship
Loudness depends on amplitude, while pitch depends on frequency. A flute produces high-frequency, low-amplitude sound, while a drum produces low-frequency, high-amplitude sound.
Audible and Inaudible Sounds
The human ear can detect sound frequencies between 20 Hz and 20,000 Hz.
-
Sounds below 20 Hz are called infrasonic.
-
Sounds above 20,000 Hz are called ultrasonic.
Animals like dogs and bats can hear ultrasonic sounds that humans cannot. Bats use ultrasonic waves for navigation (echolocation), and dolphins use them to communicate underwater.
Reflection of Sound
Just like light, sound also reflects when it hits a hard surface such as a wall, mountain, or building.
The law of reflection applies to sound as well:
-
The angle of incidence = angle of reflection.
You can experience this when you shout in an empty hall or valley and hear your voice returning after a short delay — this is called an echo.
Echo
An echo is the repetition of a sound due to its reflection from a distant surface.
To hear an echo clearly, the reflected sound must reach the ear at least 0.1 seconds after the original sound.
Given that sound travels about 340 m/s, the reflecting surface should be at least 17 meters away.
Uses of Echo:
-
SONAR: Used by submarines to detect underwater objects.
-
Ultrasound: Used by doctors to view internal organs.
-
Bats and dolphins: Use echo to locate objects and prey.
Reverberation
In a large hall, sound waves reflect many times before fading. The multiple reflections cause the sound to persist even after the source stops. This phenomenon is called reverberation.
Reverberation makes speech or music unclear in auditoriums. To avoid it, walls and ceilings are covered with sound-absorbing materials like carpets, curtains, or foam panels.
Music and Noise
Every sound is not pleasant to hear. The nature of vibrations determines whether the sound is musical or noise.
-
Musical sound: Regular and periodic vibrations (like instruments).
-
Noise: Irregular, non-repetitive vibrations (like honking or shouting).
Excessive noise causes noise pollution, which can harm hearing and cause stress, lack of sleep, and hypertension.
Noise Pollution
Noise pollution is one of the modern world’s biggest issues. It’s caused by excessive and unwanted sounds in the environment.
Main Sources:
-
Vehicle horns and traffic.
-
Loudspeakers and fireworks.
-
Industrial machines.
-
Construction activities.
Harmful Effects:
-
Temporary or permanent hearing loss.
-
Lack of concentration and sleep.
-
Increased stress and blood pressure.
-
Disturbance to wildlife and communication.
Ways to Control Noise Pollution:
-
Use silencers in vehicles and machines.
-
Avoid using loudspeakers late at night.
-
Plant trees — they absorb sound.
-
Soundproof homes and offices with thick curtains and carpets.
-
Maintain distance between residential and industrial areas.
Structure of the Human Ear
Our ear is a marvelous organ that helps us hear and balance. It converts sound energy into electrical signals that our brain interprets as sound.
Parts of the Ear:
-
Outer Ear: The visible part called the pinna collects sound waves and directs them through the ear canal to the eardrum.
-
Middle Ear: It contains three tiny bones — the hammer, anvil, and stirrup — which amplify the vibrations from the eardrum.
-
Inner Ear: The vibrations reach a coiled tube called the cochlea, which converts them into electrical signals. These signals are sent to the brain through the auditory nerve, allowing us to hear.
Working of the Ear
When sound waves reach the ear:
-
The pinna collects them and sends them through the ear canal.
-
They hit the eardrum, making it vibrate.
-
The vibrations pass through the ossicles (tiny bones).
-
These vibrations then move the fluid in the cochlea, stimulating hair cells.
-
The auditory nerve sends the signals to the brain, which recognizes the sound.
This entire process happens in less than a second, allowing us to perceive sound instantly.
Applications of Sound in Daily Life
-
Communication: Speech, music, alarms, and instruments all use sound to convey messages.
-
Medical Uses:
-
Ultrasound helps examine organs and tissues inside the body.
-
Echocardiography checks the heart’s movement.
-
-
Navigation: SONAR uses reflected sound waves to measure underwater distances.
-
Cleaning: Ultrasonic cleaners remove dirt from delicate items like jewelry or lenses.
-
Security: Some alarm systems use ultrasonic sensors for motion detection.
Sound in Musical Instruments
Each instrument produces sound in a unique way:
-
String instruments (guitar, violin): Vibrating strings.
-
Wind instruments (flute, trumpet): Vibrating air columns.
-
Percussion instruments (drum, tabla): Vibrating membranes.
The material, shape, and size of the vibrating part affect the pitch and tone of the sound.
Fun Experiments
-
Ruler Experiment: Place a ruler on the edge of a table and flick it. Shorter lengths produce higher-pitched sounds — showing how frequency changes with length.
-
Balloon Experiment: Blow air into a balloon and tap it — the sound produced changes with air pressure, showing how vibration frequency affects sound.
-
Rubber Band Guitar: Stretch a rubber band on a box. Thicker bands produce deeper sounds, thinner bands produce higher ones.
Scientific Facts about Sound
-
The speed of sound in air is 343 m/s at room temperature.
-
Whales can communicate across hundreds of kilometers underwater using low-frequency sound waves.
-
Light travels faster than sound — that’s why we see lightning before hearing thunder.
-
Dogs can hear sounds up to 45,000 Hz.
-
Sound waves can be used to break kidney stones (lithotripsy).
Conclusion
Sound is not just something we hear — it’s a bridge of communication and life itself. It carries information, emotion, and identity. The science of sound connects physics, biology, music, and technology in fascinating ways.
From the simple vibrations of a drum to the precise ultrasound used in hospitals, sound shapes our world in ways we often take for granted. Understanding how sound travels, behaves, and interacts with matter helps us protect our ears, create better instruments, and live in a quieter, healthier environment.
