Mechanical Wave

Key Concepts

1. Medium
2. Propagation of sound
3. Sound waves are mechanical waves
4. Water waves and Jump rope waves

Introduction: 

When a musician is performing on the stage the audience listen to the music, enjoy and cheer the musician. Here the sound produced by the musician reaches the audience. This sound is produced by the vibration of a part of the musical instrument and reaches the ears of the audience, making them hear the music. In this section we will be looking at the way in which the music travels from the musical instrument to the ears of the audience.  

Explanation: 

Propagation of Sound and Medium: 

The sound moves from the source to the destination. The process of sound traveling from one place to the other is called the propagation of sound. The substance through which sound is transmitted is called a medium. In the case mentioned in the introduction the medium of transmission of sound is air. Sound moves through a medium from its point of generation to the listeners. A medium can be a solid, a liquid or a gas. It consists of tiny particles such as atoms, molecules, dust etc. When sound travels through a medium, it disturbs the particles of the medium. This disturbance generates from a source of sound, passes through the medium and reaches the listeners.  

Sound – A mechanical wave: 

Sound disturbs the particles of the medium in which it travels through. A wave is a disturbance that travels through a medium, transporting energy from one place to another. Thus, sound is a wave as it travels through the medium as a disturbance, carrying energy with it from the source to the destination. 

Sound cannot travel through a medium which has no particles in it such as vacuum. Sound needs a material medium for its propagation. For example, air, water, metals, steel etc. Sound cannot travel through a vacuum, as it has no particles. This is because, in the absence of particles in a medium the disturbance cannot propagate through it. Such waves which require a material medium for their travel from one place to another are called mechanical waves. Thus, sound waves are mechanical waves. 

Bell jar experiment: 

The mechanical nature of sound waves can be tested by conducting an experiment. The experimental setup consists of an airtight glass bell jar and electric bell. The bell jar is connected to a vacuum pump and the electric bell is suspended inside it. If the bell is turned on, the ringing sound will be clearly audible. Suppose the vacuum pump is now turned on. The air inside the bell jar is gradually pumped out of it. When the vacuum pump is now turned on. The air inside the bell jar is gradually pumped out of it. Thus, there are lesser and lesser number of air particles available with time for the propagation of sound in the jar. Therefore, the sound of the ringing bell becomes fainter and fainter even though the same amount of current passes through the bell. If the air is completely removed from the jar, no sound will be heard even if the bell is on. 

Water waves and jump rope waves – mechanical waves: 

When a stone is thrown into a still pond water, a disturbance is created as circular ripples around the point where the stone hits the water surface. This disturbance travels from that point towards the end. Those ripples are called water waves. 

The jump rope waves are mechanical waves, as they require the material particles of the rope for their propagation. 

Summary:

1. The process of sound traveling from one place to the other is called the propagation of sound.
2. The substance through which sound is transmitted is called a medium. It can be a solid, a liquid or a gas.
3. When sound travels through a medium, it disturbs the particles of the medium. This disturbance generates from a source of sound, passes through the medium and reaches the listeners.
4. A wave is a disturbance that travels through a medium, transporting energy from one place to another. Thus, sound is a wave.
5. Sound cannot travel through a medium which has no particles in it such as vacuum.
6. Such waves which require a material medium for their travel from one place to another are called mechanical waves.
7. Sound waves, water waves and jump rope waves are mechanical waves as they require a material medium for their propagation.