Class 9 Science

# Compression and Rarefaction

• Compression and rarefaction in slinky
• Compression and rarefaction in air
• Density, pressure and disturbance
• Production of sound in laboratory

You have read that sound waves travel by creating disturbance in particles of a medium. During propagation of sound waves, particles of medium come closer to each other and then move away from each other.

When particles of medium come closer to each other, it is called compression of particles. When particles of medium move away from each other, it is called rarefaction of particles. Compressions and rarefactions are formed alternately in any region of particles through which a wave travels.

## Compression and Rarefaction in Slinky

When a slinky is jerked to and fro, two phenomena take place simultaneously. Some of the coils come closer to each other while some of the adjacent coils go farther from each other. This phenomenon continues and the wave moves forward.

When some of the coils come closer to each other, it is called compression. When some of the coils move away from each other, it is called rarefaction.

In the given figure the area of compression is denoted by letter C and the area of rarefaction is denoted by letter R.

## Compression and Rarefaction in Air

When sound wave travels through a medium, say air, the particles of medium are disturbed in the same fashion, i.e. compression and rarefaction. Air particles come closer because of compression. On the other hand, air particles move away from each other because of rarefaction.

This is similar to the compression and rarefaction produced in the slinky. In case of compression, molecules of medium come closer to each other and in case of rarefaction, molecules of medium go awat from each other.

## Density, pressure and disturbance

When compression takes place in a medium, the density and pressure of the medium increase. When rarefaction takes place in the medium, density and pressure of the medium decrease. Such increases and decreases in density and pressure are temporary in nature.

Compression is called the region of high density and pressure. Rarefaction is called the region of low density and pressure.

## Production of sound in laboratory

In laboratories, sound wave is produced by striking a tuning fork over a rubber pad. When prongs of the tuning fork are struck on a rubber pad, the prongs start vibrating and produce sound.

One can feel the vibration of prongs by touching them after striking on the rubber pad. The vibration produces sound energy. This sound energy creates disturbance in the medium by making compressions and rarefactions, and the sound wave propagates forward.