Sound | Science Grade 9 CBSE

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The language of sound

Six words turn a noise into physics. Tap each term to see what it measures and how it shapes what you hear.

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The three big ideas

A vibrating object pushes the particles of the medium back and forth, sending out a wave of energy — not matter.
Sound is a longitudinal wave: particles vibrate along the direction the wave travels, making compressions (squeezed-together regions) and rarefactions (spread-out regions).
Sound needs a material medium — solid, liquid or gas. There is no sound in a vacuum, because there are no particles to pass the vibration on.

Common mistake: assuming sound travels everywhere like light does. Sound cannot travel through a vacuum — it always needs a medium, whereas light (an electromagnetic wave) crosses empty space easily. That is why space is silent.

Frequency (f) = vibrations per second, in hertz (Hz). It sets the pitch — higher frequency, higher pitch.
Amplitude = the size of the vibration. It sets the loudness — bigger amplitude, louder sound.
Wavelength (λ) = the distance between two consecutive compressions (or rarefactions).
Speed links them: v = f × λ. Sound is fastest in solids and slowest in gases.

Worked example. A sound has frequency 500 Hz and wavelength 0.66 m. Find its speed.

v = f × λ = 500 × 0.66 = 330 m/s — close to the speed of sound in air.

Sound bounces off hard surfaces. An echo is a sound heard again after it reflects off a surface and returns to you.
Reverberation is the repeated reflection that makes a sound persist in a closed hall; too much of it is reduced with soft, absorbing materials.
Reflection is put to work: SONAR uses echoes of ultrasound to measure sea depth and find objects; ultrasound is used for medical scans and cleaning.

Where you'll meet it

Sound, put to work

SONAR maps the sea

A ship sends an ultrasound pulse straight down and times the echo from the seabed. Since depth = (speed of sound in water × time) ÷ 2, the echo's travel time reveals exactly how deep the water is.

Ultrasound scans the body

High-frequency sound (above human hearing) is sent into the body and its echoes are turned into an image — letting doctors see an unborn baby or check organs without any cut or X-ray.

Check yourself

Competency quiz

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

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