Notes 12. Sound - Propagation of Sound | Class 9 Science - Toppers Study

12. Sound 

Sound: When an object vibrates it produces sound. 

Vibration: When an object moves fast in to and fro motion, this motion is called vibration. 

Ways to produce sound:

(i) Hitting on an object.

(ii) By friction

(iii) By scratching

(iv) By rubbing

(v) By blowing or moving any object sound can be produced. 

Sound in Human:

In Human being sound is produced by vibrating the vocal chords.

• Sound moves in form of waves. 
• Honeybee is produced sound by vibration of its feathers, this is called humming.   

Propagation of Sound: The sound travels from one place to another place, this is called propagation of sound. 

Medium: Sound does not propagate without any medium. It need medium to travel from one to another place. "substances like solid, liquid or gas provides medium to sound for propagation, such substances are called mediums." 

Definition of medium: The matter or substance through which sound is transmitted is called a medium.

Medium of Sound for Propagation: 

There are three medium by which sound propagates.

(i) Solid like metalic rod, glass, wood etc.

(ii) Liquid like water, oil and drinks etc. 

(iii) Gas like air, and other gases. 

• Sound does not propagte in vaccum because it has not any particles of medium. 
• The medium which has high density, the sound moves very fast or the speed of sound is greater in that medium.
• So that the speed of sound is the greatest in solid inspite of all mediums. 

How does the sound propagates?

When an object vibrates, it sets the particles of the medium around it vibrating. A particle of the medium in contact with the vibrating
object is first displaced from its equilibrium position. It then exerts a force on the adjacent particle. As a result of which the adjacent particle gets displaced from its position of rest. After displacing the adjacent particle the first particle comes back to its original position. This process continues in the medium till the sound reaches our ear.

Compression: When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. This region is called a compression(C). 

Rarefaction: When the vibrating object moves backwards, it creates a region of low pressure called rarefaction (R).

Differences between compression and rarefaction: 

Compression:

(i) It creates when object moves forward.

(ii) Compression is the region of high pressure.

Rarefaction: 

(i) It creates when objects moves backward.

(ii) Rarefaction is the region of low pressure.

Propagation of density variations or pressure variations: 

Pressure is related to the number of particles of a medium in a given volume. More density of the particles in the medium gives more pressure and vice versa. Thus, propagation of sound can be visualised as propagation of density variations or pressure variations in the medium.