When sound waves travel in medium, the particles of the medium vibrate about their equilibrium positions along the direction of the propagation of the waves.

**Sound Waves: Longitudinal Waves**

**Introduction**
Sound is a form of energy that propagates through various mediums, such as air, water, or solids, in the form of waves. These waves are known as sound waves. Sound waves are classified as longitudinal waves due to the nature of their particle displacement and the direction of wave propagation.

**Understanding Longitudinal Waves**
To understand why sound waves are called longitudinal waves, it is important to comprehend the characteristics of longitudinal waves in general. Longitudinal waves are a type of mechanical waves in which the particles of the medium vibrate parallel to the direction of wave propagation. This is in contrast to transverse waves, where particles oscillate perpendicular to the direction of wave motion.

**Particle Displacement in Sound Waves**
In sound waves, the particles of the medium (such as air molecules) oscillate back and forth in the same direction as the wave propagates. As a sound wave travels through a medium, it creates regions of compression and rarefaction. In the compression regions, the particles are pushed close together, resulting in high-pressure areas. Conversely, in the rarefaction regions, the particles move apart, leading to low-pressure areas.

**Visualization of Longitudinal Waves**
To visualize a longitudinal sound wave, imagine a slinky toy stretched out horizontally. If you push one end of the slinky forward and backward, a compression and rarefaction pattern will be created. The adjacent coils of the slinky will experience alternating regions of compression and rarefaction, similar to how air molecules behave in a sound wave.

**Propagation of Longitudinal Waves**
Sound waves propagate through a medium by transferring energy from one particle to the next. As particles vibrate, they transfer this energy to neighboring particles, causing a chain reaction. This transfer of energy from particle to particle enables the sound wave to travel through the medium.

**Conclusion**
In conclusion, sound waves are classified as longitudinal waves because they exhibit particle displacement parallel to the direction of wave propagation. The oscillation of particles in a medium creates regions of compression and rarefaction, leading to the formation of sound waves. Understanding the nature of sound waves as longitudinal waves helps explain their behavior and characteristics, such as the speed of sound, frequency, and wavelength.