Sound Summary Class 8 NCERT Summary Chapter 11

Introduction

• 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

• 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.

Noise

• 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.