Propagation of Sound

Key Concepts

• Propagation of sound
• Compression and rarefaction

 

Introduction: 

Sound starts from a vibrating object, propagates through a material medium and then reaches the listeners ears. While propagating through a medium the sound waves disturb the particles of the medium. In this section we will be looking at the way in which the particles are disturbed by the propagation of sound through them. 

Explanation: 

Propagation of Sound: 

Sound is generated when an object vibrates. These vibrations set the neighboring particles of the medium to vibration. These vibrating particles set their neighboring particles to vibration. This process continues till the vibrations are carried to the ears of the listener. The following points refer to the way in which the sound propagates through a medium. 

• The particles of the medium which is in contact with the vibrating object is first displaced from its equilibrium position. 
• This vibrating particle hits the adjacent particle, which gets displaced from its equilibrium position and starts vibrating. After displacing the adjacent particle, the first particle comes back to its original position. 
• This particle in turn sets the next particle to vibration and then comes to rest.  

• This process repeats itself until it reaches the particle near the ear of the listener. 
• This disturbance created by a source of sound in the medium travels through the medium and not the particles of the medium. 
• The particles of the medium do not themselves move forward, but the disturbance is carried forward. 

Activity: 

The following activity may be conducted to visualize the way in which the sound propagates through a medium by disturbing the particles. 

• For this we need a slinky and fix one of its ends to a rigid object such as a wall and its body rests on a table. 
• Now, hold the free end of the slinky and push it forward. 
• It will be observed that some of the turns of the slinky come closer together and form a group. 

• This group of turns then moves forward towards the fixed end of the slinky. 
• The movement of the sound wave in the air takes place in a similar fashion. 

Propagation of sound: 

• When an object vibrates in the air medium, it pushes the nearby air particles and compresses them while moving forward. 
• This creates a high-pressure or high-density region called compression (C). 

• This compression then starts to move away from the vibrating object. 
• Now when the vibrating object moves backward it creates a region of low pressure or low density called the rarefaction (R). 
• As the object vibrates it moves back and forth multiple times. 
• Every time it vibrates it creates one region each of compression (C) and rarefaction (R). 
• As a result of multiple vibrations, a series of Cs and Rs are generated in the air. 

• These Cs and Rs form the sound wave which moves through the medium. 
• Compressions are the regions of high pressure and has a high density of particles. 
• Rarefactions are the regions of low pressure and has a low density of particles. 
• The sound wave is propagated through the medium in terms of the propagation of the compressions and rarefactions. 
• Thus, propagation of sound can be visualized as the propagation of density or pressure variations in the medium. 

Summary:

• The process of sound traveling from one place to the other is called the propagation of sound.
• When an object vibrates in a medium the particles of the medium which are in contact with the it first get displaced from their equilibrium positions.
• These particles disturb their neighboring particles in a similar way, after which they come to rest.
• This disturbance created by a source of sound in the medium travels through the medium and not the particles of the medium.
• The sound wave is propagated through the medium in terms of the propagation of the compressions and rarefactions.
• The propagation of sound can be visualized as the propagation of density or pressure variations in the medium.