Concepts

Class 9
Chapter 12 Class 9 - Sound

## What is general meaning of word Compression?

It is a region of high pressure

Example

When a vibrating object moves forward in air as medium,

it pushes and compresses the air in front of it

creating a region of high pressure. This region is called a compression.

## What is general meaning of word Rarefaction

It is a region of low pressure

Example

When a vibrating object moves backward in air as medium,

it creates a region of low pressure. This region is called a rarefaction.

## Difference between Compression and Rarefaction

 Compression Refraction It is a region of high pressure It is a region of low pressure Air particles are closely packed together Air particles are loosely packed

## How does Sound Waves cause Compression and Rarefaction

We know that Sound waves travel through a medium like air

When an object vibrates, it moves in to and fro motion (forward and backward)

1. When object moves in forward motion
It causes nearby air particles to compress
This creates a region of high pressure called compression

2. When object moves back in backward motion
It creates a region of low pressure
This creates a region of low pressure called rarefaction

As the object vibrates (moves backward and forward), a series of compression's and rarefaction's is created in the air.

These make the sound waves propagate through the medium

## Questions

Intext Question 1 Page 163 - Explain how sound is produced by your school bell.

NCERT Question 2 - Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound.