Propogation of Sound

→ The substance through which sound travels is called a medium.

→ The medium may be solid, liquid or gas.

→ When an object vibrates, then the air particles around it also start vibrating in exactly the same

way and displaced from their stable position.

→ These vibrating air particles exert a force on nearby air particles so they are also displaced from

their rest position and start to vibrate.

→ This process is continued in the medium till sound reaches our ears.

→ The disturbance produced by sound travels through the medium (not the particles of the


→ Wave is a disturbance which travels through a medium and carries energy.

→ So sound travels in wave form known as mechanical waves.

• When a body vibrates then it compresses the air surrounding it and form a area of high density

called compression (C).

Compression is the part of wave in which particles of the medium are closer to one another

forming high pressure.

→ This compression move away from the vibrating body.

• When vibrating body vibrates back a area of low pressure is formed called rarefaction (R).

Rarefaction is the area of wave in which particles of the medium are further apart from one

another forming a low pressure or low density area.

→ When body vibrates back and forth, a series of compression and rarefaction is formed in air

resulting in sound wave.

→ Propogation of sound wave is propogation of density change.

Sound needs Medium for Propogation

• Sound waves are mechanical waves.

→ It needs material medium for propogation like air, water, steel etc.

→ It cannot travel in vaccum.

→ An electric bell is suspended in airtight bell jar connected with vacuum pump.

→ So, medium is necessary for propagation of sound.

Experiment to show that sound cannot travel through vacuum