Notes of Ch 13 Sound| Class 8th Science

Study Material and Notes of Ch 13 Sound Class 8th Science

Topics in the chapter

• Introduction
• Vibration motion
• How sound is produced by Humans
• Propagation of Sound
• Frequency
• Loudness of Sound
→ Higher amplitude
→ Pitch
→ High frequency
→ Audible sound
• Noise
• Noise pollution
→ Measures to control noise pollution
• Human ear


→ Sound helps us to communicate with one another and plays an important role in our life.

→ Vibrating body produces sound.

→ In this chapter, we will know about how sound is produced and how we hear them.

Vibration motion

→ to-and-fro or back-and-forth or up-and-down motion of a body.

→ Sound is a form of energy that is produced by producing vibration in an object.

→ Sound cannot move through vacuum.

→ Sound waves are longitudinal waves.

How sound is produced by Humans?

→ In humans, sound is produced by voice box or larynx.

→ When the lungs force air through the slit, the vocal cord vibrates which produce sound.

→ Sometimes, the amplitude of produced sound is so small that we can't hear them.

→ The muscles attached to the vocal cords can make the cords tight or loose to produce different types of sounds.

→ Different people have different vocal chords. Due to this reason, we all have a different voice quality.

Propagation of Sound

→ Sound requires material medium for propagation.

→ Sound can travel through solid, liquid or gas.

→ Sound cannot travel through vacuum.

→ No sound can be heard in outer spaces.


→ Frequency is measure of number of oscillations per second.

→ It is measured in hertz(Hz).

Loudness of sound

→ It is measured in decibel (dB).

→ It depends on amplitude.

• Higher amplitude– louder sound

Pitch or shrillness depends on frequency.

• Higher frequency– higher pitch

• Audible sound– Human ear can hear sounds having frequency in the range of 20-20,000 Hz.


→ Unpleasant sound is called noise.

Noise pollution

→ Presence of unwanted and excessive sound in the environment
→ Noise pollution may cause many health related problems.

Measures to control noise pollution

→ Moving noise producing industries away from residential area.

→ Minimizing the usage of loud speakers.

→ Avoiding unnecessary usage of horns

→ Planting more and more trees.

Human ear

→ Human ear has three parts outer, middle and inner.

→ Shape of the outer part of the ear is like a funnel.

→ In human ear, the eardrum vibrates and passes vibration to the inner ear.

→ The eardrum is like a stretched rubber sheet.

→ Sound vibrations make the eardrum vibrate, from there the signal goes to the brain.

→ Noise level of 85 dB can damage the human ear.

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