Short Notes and Formulas of class 8 science chapter-Sound

Short Notes and Formulas of class 8 science chapter-Sound

May 16, 2022, 16:45 IST

Class- 8 science chapter-Sound

Sound is a form of energy which makes us hear and travels in the form of waves. Sound is produced by various sources. Though sound is mechanical in nature, its perception is, of course, largely physiological.

Propagation of Sound

The sound produced by vibrating objects reaches the listener only when it passes through a medium which may be a liquid, a solid or a gas. Let us now briefly discuss as to how sound travels from the source point of generation to the listener.

1. The vibration of the object sets the particles of the medium around it vibrating.
2. The particles do not travel from the vibrating object to the ear. A particle of the medium in contact with the vibrating object is first displaced from its equilibrium position as shown in the figure. This particle exerts a force on a neighbouring particle (2) which is displaced from its equilibrium position and also starts moving. After displacing particle (2), particle (1) comes back to its mean position. This process continues till the particle near the listener's ear starts vibrating.

Thus, sound created by the source reaches the listener through the particles of the medium without any net transport of the medium

(a) Type of waves on the basis of material medium

• Elastic waves or mechanical waves : Those waves which need a material medium for their propagation are called elastic waves or mechanical waves e.g., sound waves and     water waves are elastic or mechanical waves.
• Electromagnetic waves: Those waves which do not need a material medium for their propagation are called electromagnetic waves . These waves can travel through vacuum as well as through medium e.g. light waves and radio waves are electromagnetic waves.
• Seismic waves: The disturbances inside the earth which cause waves moving in all direction are called seismic waves. It is these waves which cause earthquakes.

(b) Types of waves on the basis of its direction of propagation

• Transverse waves: The waves in which the particles of the medium vibrate up and down at night angles to the direction in which the wave is moving are called transverse waves . Eg., light waves, radio waves, water waves etc.

The water waves (or ripples) formed on the surface of water in a pond are transverse waves. This is because of the fact that in a water wave, the molecules of water move up and down in the vertical direction when the wave travels in the horizontal direction along the water surface. When a stone is dropped in a pond of water, transverse water waves are produced on the surface of water. The ‘elevation' or 'hump' in a transverse wave is called crest . In other words, a crest is that part of the transverse wave which is above the line of zero disturbance of the medium. In figure, XY is the line of zero disturbance and A and C are the two crests of the transverse water waves.

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