Class 9 Science


Sound is a type of energy. Sound travels in the form of wave from one place to another.

Production of sound:

Sound is produced because of the vibration of any object. In other words, when an object is vibrated it produces sound.


Thus, an object is needed which could be vibrated to produce sound.

Table of Contents

Propagation of Sound

When a stone is dropped in pond water, it produces many circular wrinkles and water appears to be travelling outwards from centre. These circular wrinkles are called ripples. This happens because of formation of waves in water.

When a stone is dropped in water, it creates a disturbance in water particles. The water particles pass the disturbance to the adjacent particles and the adjacent particles pass the disturbance to the next adjacent particles and so on. This phenomenon continues and the disturbance moves outward from the centre of disturbance. This makes water appear to be moving. In fact, water molecules do not travel in this case, rather only the disturbance is passed through particles of water. This creates wave in the form of ripples in water.

Sound travels from one place to another in similar manner i.e. through wave formation. In this case, the particles of medium do not travel rather only the disturbance; caused by sound energy; passes to the adjacent particles of the medium. So, the traveling of sound is called propagation of sound.

Thus, sound propagates from one place to another in the form of waves, i.e. because of the disturbance of particles of the medium.

Sound Wave

The disturbance or oscillation from one location to another location; accompanied by transfer of energy is called wave.

There are two types of wave, viz. Electromagnetic Wave and Mechanical Wave.

Electromagnetic Wave

Wave that requires no medium to propagate is called Electromagnetic wave. For example; light wave. Light can also travel through vacuum.

Mechanical Wave

Wave that requires medium to propagate is called Mechanical Wave. For example - sound wave. Sound cannot travel in the absence of a medium.


On the basis of direction of propagation, waves can be divided into two types, viz. transverse wave and longitudinal wave.


The wave formed because of the oscillation perpendicular to the disturbance is called transverse wave. For example light wave, water wave, etc.

transverse wave

In transverse wave, particles oscillate in up and down directions; relative their normal position of rest. Particles of medium create elevation above the normal position or line of zero to the surface of medium and depression below the normal position in the course of oscillation. The elevation is called crest and the depression is called trough.

crest and trough of sound wave


The wave formed because of the oscillation; parallel to the disturbance; is called longitudinal wave. For example; sound waves. ause of forward and backward movement of particles of the medium. If a slinky is pushed and pulled backward and forward, the wave formed in slinky is similar to longitudinal wave.

longitudinal wave in slinky

Sound Wave is longitudinal wave. Sound propagates because of oscillation of particles of medium parallel to the disturbance, thus sound wave is longitudinal wave.

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