Why is Sound Wave Called a Longitudinal Wave? – Check Key Reasons, Characteristics and Explanation

Why is Sound Wave Called a Longitudinal Wave?

Sound Waves are called longitudinal waves because they propagate through a medium by causing particles of the medium to move in a direction parallel to the direction of the wave itself. They oscillate parallel to the direction of propagation, forming the compressions and rarefactions. This motion of particles is what distinguishes longitudinal waves from other types of waves, such as transverse waves. The following are key reasons why sound waves are called longitudinal waves – 

1. Direction of Particle Displacement – In a longitudinal wave, the individual particles of the medium (whether it's air, water, or a solid material) move back and forth in a direction parallel to the propagation of the wave. This means that when a sound wave travels through air, for example, the air particles vibrate or oscillate along the same line that the sound wave is moving. Imagine compressing and rarefying a slinky in the same direction the wave is traveling; this is a good analogy for a longitudinal wave.
2. Compression and Rarefaction – As a sound wave passes through a medium, it creates areas of compression and rarefaction. In the compression phase, particles are pushed closer together, while in the rarefaction phase, they move farther apart. These regions alternate as the wave progresses. This behavior results in the characteristic pattern of compressions and rarefactions that we associate with sound waves.
3. Propagation Mechanism – Longitudinal waves are generated by a disturbance or vibration at a source, such as a vibrating object. The energy from this source is transferred through the medium by the successive collisions and interactions between neighboring particles. As one particle moves, it collides with its neighboring particles, causing them to also move in a longitudinal fashion. This domino effect of particle displacements carries the energy and information of the sound wave from the source to our ears, allowing us to hear the sound.
4. Examples of Longitudinal Waves – Besides sound waves, there are other examples of longitudinal waves. Earthquakes generate longitudinal waves known as Primary or P-waves, which travel through the Earth. In a slinky or spring, mechanical waves produced when you push or pull it in a direction parallel to its length are also longitudinal waves.
5. Contrast with Transverse Waves – It is important to differentiate longitudinal waves from transverse waves. In a transverse wave, the particles of the medium move perpendicular to the direction of wave propagation. A classic example of a transverse wave is a water wave, where the water particles move up and down, while the wave moves horizontally.

 

Therefore, sound waves are called longitudinal waves because they involve the parallel displacement of particles in the medium. This unique movement of particles sets sound waves apart from transverse waves, and it is this specific motion that allows sound to travel through various mediums, enabling us to hear and perceive the world through auditory sensations.

Also read – Waves and its Types and Properties

What is a Longitudinal Wave?

A longitudinal wave is a wave in which the oscillations of the medium are in the same direction as the propagation of the wave. In a longitudinal wave, the displacement of the medium is parallel to the direction of the wave propagation. The particles of the medium vibrate back and forth along the axis of the wave propagation. When a longitudinal wave passes through a medium, it causes compressions and rarefactions of the medium along the axis of the wave motion. Let us understand this with an example - when a slinky (a toy spring) is stretched and compressed to produce waves, the coils or rings of the slinky move back and forth along the length of the slinky. This type of motion generates a longitudinal wave. The particles do not move perpendicular but along the length of propagation.

Read what are Longitudinal Waves in detail.

Characteristics of Longitudinal Waves

The following are the key characteristics of longitudinal waves – 

• Vibrations are parallel to the direction of propagation – In longitudinal waves, the vibrations or oscillations of the medium particles take place along the direction of wave propagation. The particles vibrate back and forth along the same axis that the wave travels.
• Medium oscillates along the axis of propagation – As the longitudinal wave passes through a medium, the particles of that medium do not oscillate or vibrate perpendicular to the wave propagation direction. Rather, the particles oscillate back and forth along the longitudinal axis parallel to the direction in which the wave is traveling.
• Causes compressions and rarefactions in the medium – Areas of increased particle density or pressure are created which are called compressions, while areas of decreased particle density or pressure are called rarefactions. The vibrations of particles cause a squeezing together (compression) or spreading out (rarefaction) effect along the propagation axis.
• No transverse motion of the medium – Unlike transverse waves, longitudinal waves do not exhibit a motion perpendicular to the propagation direction. The vibrations are constrained only along the axis parallel to the direction the wave travels through the medium.
• Examples include sound waves and seismic P-waves – Common longitudinal wave types are sound waves propagating through gases, liquids and solids. Another example is P-waves or primary waves generated during earthquakes that move by compressing and decompressing the Earth.
• Lower velocity as compared to transverse waves – In general, longitudinal waves travel slower than transverse waves through a given medium. This is because particle oscillations aligned with propagation induce less motion than perpendicular oscillations in a transverse wave.

Also read – Types of Waves

Why are Sound Waves Called Longitudinal Waves?

Sound waves are a type of mechanical wave that requires a medium for propagation. In gases, liquids, and solids, the particles of the medium can vibrate and transmit this vibration from one particle to the next, thereby transferring the energy of sound waves. When a sound wave passes through a medium like air, the particles of the medium vibrate parallel to the direction of the propagation of the wave due to changes in pressure. The particles in the air move back and forth along the axis of propagation. This motion makes sound waves exhibit the defining characteristics of a longitudinal wave like –

• Vibrations are parallel to the propagation direction
• Medium oscillates along the axis of propagation
• Causes compressions and rarefactions in the medium

In a sound wave, the regions of high pressure are called compressions and the regions of low pressure are called rarefactions. The particles of the medium move in and out along the propagation axis which results in these alternating regions of compression and rarefaction.

This clearly shows that sound waves can only be longitudinal since the vibrations have to be parallel to transfer the energy down the propagation path. Hence, sound waves are rightly called longitudinal waves based on their wave propagation behavior in gases, liquids and solids.

Are Sound Waves Longitudinal or Transverse Waves?

Before understanding whether sound waves are longitudinal waves or transverse waves, let us look at the difference between the two types of waves –

As illustrated above, it is clear that sound waves exhibit all the characteristics of longitudinal waves, which include –

• Vibrations parallel to propagation direction
• Medium oscillates along the axis of propagation
• Causes compressions and rarefactions

On the other hand, sound waves do not show any traits of Transverse Waves such as vibrations perpendicular to the propagation direction or lack of compressions/rarefactions.

Therefore, to conclusively answer - sound waves are longitudinal waves and not transverse waves. The medium particles undergoing vibration for sound wave propagation can only move along the longitudinal axis in gases, liquids or solids to transfer sound energy.

Did you find this article on “Why is Sound Wave Called a Longitudinal Wave?” to be useful and informative in understanding the concept in detail? If yes, download the Testbook App now to get access to industry-level learning resources, expert coaching, quizzes, mock tests and study materials to ace your preparation for competitive exams.