Sound | Science Grade 8 CBSE

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The six ideas of sound

From a single vibration to noise pollution — tap each term to see what it means and how it shapes what you hear.

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The three big ideas

Every sound is made by a vibrating object — a plucked sitar string, a beaten tabla skin, a ringing bell, or our own vocal cords.
The vibration shakes the particles of the surrounding medium (air, water or a solid), passing the disturbance along until it reaches our ear.
Sound travels through solids, liquids and gases — and usually faster in solids than in air — but it cannot travel through a vacuum, because there are no particles to carry it.
In the ear, the sound makes the eardrum vibrate; the brain reads these vibrations as sound.

Common mistake: thinking sound can travel through empty space like light. Without a medium there is no sound — which is why space is silent and astronauts use radios.

Frequency is the number of vibrations per second, measured in hertz (Hz). It decides the pitch: higher frequency → higher pitch (a whistle); lower frequency → lower pitch (a drum).
Amplitude is the size of the vibration. It decides the loudness: a bigger amplitude → a louder sound. Loudness is measured in decibels (dB).
Pitch and loudness are independent — you can have a high soft note or a low loud one.

Worked example. You strike a tabla harder. What changes — pitch, loudness, or both?

A harder strike makes the skin vibrate with a larger amplitude, so the sound is louder. The frequency (and so the pitch) barely changes — that is set by how tight and big the drum is.

The healthy human ear hears roughly 20 Hz to 20,000 Hz — this is the audible range. Sounds below or above it are inaudible to us, though some animals can hear them.
Musical sound is pleasant and regular; noise is unpleasant and irregular. A very loud sound, even a musical one, can become noise.
Noise pollution — from traffic, loudspeakers, horns and machines — causes stress, sleeplessness and, over time, hearing damage.
It is reduced by planting trees, using silencers on vehicles, keeping volumes low, and shifting noisy industry away from homes and schools.

Where you'll meet it

Sound at work

Musical instruments

A sitar makes sound from vibrating strings, a flute from a vibrating column of air, and a tabla from a vibrating skin. Tightening a string or shortening the air column raises the frequency and so the pitch — the physics behind tuning every instrument.

Speaking and hearing

Air from the lungs vibrates the vocal cords in the larynx to make your voice, while your mouth and tongue shape the words. At the other end, your eardrum vibrates and the brain decodes the message — a tiny, elegant sound system.

Quieter cities

Honking bans near hospitals, green belts beside highways, and limits on loudspeaker volume are all efforts to control noise pollution — protecting hearing, sleep and concentration in crowded Indian towns.

Check yourself

Competency quiz

Modelled on the competency-based pattern — MCQ, assertion–reason and case studies, testing whether you can use the ideas, not just recall them.

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