Sound – A longitudinal Wave

Key Concepts

1. Sound – A longitudinal Wave
2. Transverse waves
3. Differentiation between longitudinal and Transverse waves

Introduction: A longitudinal Wave

Sound waves originate from a vibrating body, which in turn sets its neighboring particles of the medium into vibration. These vibrating particles set their neighboring particles in motion. This process continues until the disturbance reaches the ears of the listener. As the sound requires a material medium for its propagation, it is called a mechanical wave. We can categorize sound under one more category based on the comparison between the direction of motion of the sound wave and the direction of vibration of the particles of the medium when it propagates through the medium. 

Explanation: 

Sound – A Mechanical Wave: 

The movement of the particles of the medium when a sound wave propagates through it is shown below. 

Here, it can be noticed that the direction of vibration of particles of the medium is parallel to the direction of propagation of the sound wave. The waves in which the direction of vibration of particles of the medium of propagation is parallel to the direction of propagation of the waves are called longitudinal waves. 

Sound waves in air or any fluid medium travel by disturbing the particles in the same way. Sound waves are longitudinal waves in any fluid medium. 

Transverse waves: 

There is another category of waves wherein the direction of vibrations of the particles of the medium of propagation is perpendicular to the direction of motion of the waves. Such waves are called transverse waves. 

The ripples on the surface of water and the vibration of a string are examples of transverse waves. 

These waves are also mechanical as they involve the movement of particles of the medium, i.e., water and string, respectively.  

Some transverse waves are not mechanical in nature. It means they do not require a material medium for their propagation. Light waves come under this category. In light waves, the oscillations are not of the particles of the medium or their density or pressure, but the electric and magnetic fields. 

Activity: 

To model the way in which various waves propagate through a medium, the following activity can be performed. 

For this, a slinky is taken, and one of its ends is fixed to a rigid object such as a wall, and its body rests on a table. 

Now, the free end of the slinky is held and moved forward and backwards. The motion of the loops of the slinky is observed. 

Now, the free end of the slinky is moved up and down. The motion of the loops is observed. 

When the slinky is moved to and fro, the loops move in the way in which a longitudinal wave moves through a medium forming compressions and rarefactions. 

When the slinky is moved up and down, the loops move the way in which a transverse wave moves through a medium forming. 

Differences between longitudinal and transverse waves: 

Summary

1. The waves in which the direction of vibration of particles of the medium of propagation is parallel to the direction of propagation of the waves are called longitudinal waves.
2. Sound waves are longitudinal waves.
3. The waves in which the direction of vibration of the particles of the medium of propagation is perpendicular to the direction of propagation of the waves are called transverse waves.
4. The ripples on the surface of water and the vibration of a string are the examples of transverse waves.
5. Light waves are transverse waves which are not mechanical in nature.
6. When the free end of a slinky is moved to and fro the loops move in the way in which a longitudinal wave moves through a medium forming compressions and rarefactions.
7. When the free end of a slinky is moved up and down the loops move the way in which a transverse wave moves through a medium forming.