Class 9 Science


Compression and Rarefaction

When a slinky is jerked to and forth, two phenomena take place simultaneously. First, the coils come closer to each other while; on the other hand; some of the adjacent coils go farther from each other. This phenomenon continues and the wave goes forward.

The action when coils come closer is called compression and when coils go farther from each other is called rarefaction.

Sound formation of wave in slinky

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

When sound wave travels through a medium, say air, the particles of medium disturb in the same fashion, i.e. compression and rarefaction (depression). When air particles come closer it is called compression. On the other hand, when particles go farther than their normal position it is called rarefaction.

This is similar to the compression and rarefaction produced in the slinky. In the condition of compression, molecules of medium come closer to each other and in the condition of rarefaction, molecules of medium go farther from each other; compared to their normal positions.

Compression and rarefaction sound wave

Density, pressure and disturbance:

When compression takes place in the medium, the density and pressure of the medium increase. When rarefaction takes place in the medium, density and pressure of the medium decrease. This increase and decrease in density and pressure are temporary.

Thus, 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 over a rubber pad, the prongs of tuning fork start vibrating and produce sound.

Production of sound wave by tuning fork in laboratory

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