Sound Class 9 Science Notes

Sound Class 9 Notes CBSE Science Chapter 12

The CBSE NCERT baesd Class 9th Notes Science Revision Notes comprehensive set of Chapter 12 on Sound. These notes are an integral component of the Revision Notes for Class, and provide a succinct overview of the key concepts and topics covered in the NCERT TextBooks Class 9th Science Revision Notes . The Chapter Notes are structured in a manner that offers detailed explanations of each topic and concept, including Types of Waves, wavelength, amplitude, frequency, time period, velocity Compressions and Rarefactions, Characteristics of a sound wave, Amplitude and Wavelength.

Well prepared Class 9th Science all Chapters Notes will guidance students understand the topics ,concepts and themes covered in all the chapters of Ncert based Text book of 9th  

Sound Chapter 12 Science Notes   

The sensation felt by our ears is called sound.  Sound is a form of energy which makes us hear.  Law of conservation


The sensation felt by our ears is called sound.

Sound is a form of energy which makes us hear.

Law of conservation of energy is also applicable to sound.

Sound travels in form of wave.
Production of Sound

Sound is produced when object vibrates or sound is produced by vibrating objects.

The energy required to make an object vibrate and produce sound is provided by some outside
source (like our hand, wind etc.).
Example :
Sound of our voice is produced by vibration of two vocal cords in our throat.

Sound of a drum or table is produced by vibration of its membrane when struck.

In laboratory experiments, sound is produced by
vibrating tuning fork. The vibrations of tuning fork
can be shown by touching a small suspended pith
ball (cork ball) with a prong of the sounding tuning
fork. The pith ball is pushed away with a great force.

Sound can be produced by following methods : 
(i)By vibrating string (sitar)
(ii) By vibrating air (flute)
(iii) By vibrating membrane (table, drum)
(iv) By vibrating plate (bicycle bell)
(v) By friction in objects
(vi) By scratching or scrubbing the objects etc.

Propagation 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 medium).

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 moves 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.

Propagation of sound wave is propagation of density change. 

Sound needs Medium for Propagation

Sound waves are mechanical waves.

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

It cannot travel in vacuum.

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

When bell jar is full of air, we hear the sound but when air is pumped out from the bell jar by 
vacuum pump and we ring the bell, no sound is heard.

So, medium in necessary for propagation of sound.
Sound Waves as Longitudinal Waves

A wave in which the particles of the medium vibrate back and forth in the same direction in which 
the wave is moving, is called a longitudinal wave.

When we push and pull the slinky compression (number of turns are more or closer) an rarefaction 
(number of turns are less or farther) are formed.

When a wave travels along with slinky, its each turn moves back and forth by only a small 
distance in the direction of wave. So the wave is longitudinal.

The direction of vibrations of the particles is parallel to the direction of wave.

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