- Books Name
- Science Made Easy Science Book
- Science Made Easy
- CBSE Class 9
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