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 vibration in an object. In other words, when an object is vibrated it produces sound.

When a guitarist plays a guitar, the strings of guitar vibrate to produce sound. When musician beats his drum, the stretched membrane of the drum vibrates to produce sound.


See Answer

1: (a) Energy, 2: (d) Vacuum, 3: (b) Two, 4: (c) Vibration of membrane, 5: (d) Compression and rarefaction, 6: (b) Density decreases, 7: (c) Gap between two consecutive troughs, 8: (a) Gap between extreme positions and median, 9: (b) Meter, 10: (a) Hertz


Thus, an object is needed to vibrate to produce sound.

Propagation of Sound

When a stone is dropped in pond water, it produces many circular wrinkles and water appears to be travelling outwards from the 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. Water particles pass the disturbance to adjacent particles and adjacent particles pass the disturbance to next adjacent particles and so on. This phenomenon continues and the disturbance moves outwards from the centre of disturbance. This makes water appear to be moving. In fact, water molecules do not travel, 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 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 for propagation is called electromagnetic wave. For example: light wave. Light can also travel through vacuum.

Mechanical Wave

Wave that requires a medium for propagation is called mechanical Wave. For example: sound wave. Sound cannot travel in the absence of a medium.

Types of Sound Wave

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

Transverse Wave

transverse wave

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

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.

Longitudinal Wave

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. If a slinky is pushed and pulled backwards and forwards, 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. When sound wave travels through air, disturbance in particles and direction of wave propagation are parallel to each other.