Why is a sound wave called a longitudinal wave?

A sound wave is called a longitudinal wave because its particle motion is parallel to the direction of wave propagation. In other words, the particles in a medium (such as air, water, or solids) vibrate back and forth in the same direction that the sound wave is traveling.

Longitudinal waves are characterized by compressions and rarefactions. In a sound wave, regions of high pressure and particle density are called compressions, while regions of low pressure and reduced particle density are called rarefactions. As the sound wave propagates through the medium, these compressions and rarefactions move in a wave-like manner.

To understand this concept better, consider a slinky spring. If you hold one end of the slinky and push it back and forth, you will create a series of compressions and rarefactions that travel along the slinky. The coils of the slinky move back and forth parallel to the direction in which the wave travels. This back-and-forth motion is similar to how particles in a medium behave when a sound wave passes through it.

In contrast, a transverse wave, such as a wave on a string or an electromagnetic wave, exhibits particle motion perpendicular to the direction of wave propagation. In a transverse wave, the particles move up and down or side to side while the wave moves forward.

In summary, a sound wave is called a longitudinal wave because its particle motion is parallel to the direction of wave propagation, as opposed to the perpendicular particle motion observed in transverse waves.