give me notes of ch 12 sound class 9 physics
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Sound
Sound is a form of energy which produces a sensation of hearing in our ears.
Production of Sound
Sound is produced due to the vibration of objects.
Vibration is a periodic back-and-forth motion of the particles of an elastic body or mediumkind about a central position. It is also named as oscillation.
For example:
Stretched strings of a guitar vibrate to produce sound.
When membrane of a table is struck, it vibrates to produce sound.
Propagation of Sound
The travelling of sound is called propagation of sound.
Sound is propagated by the to and fro motion of particles of the medium.
When an object vibrates, the particles around the medium vibrate. The particle in contact with the vibrating object is first displaced from its equilibrium position. Each particle disturbs the other particle in contact. Thus, the disturbance is carried from the source to the listener.
Propagation of sound
Only the disturbance produced by the vibrating body travels through the medium but the particles do not move forward themselves.
Medium
The matter or substance through which sound is transmitted is called a medium.
A medium is necessary for the propagation of sound waves.
The medium can be solid, liquid or gas.
Sound cannot travel in vacuum.
Wave that requires medium to propagate is called Mechanical wave.
Wave
Wave is a phenomenon or disturbance in which energy is transferred from one point to another without any direct contact between them. For example: Heat, light and sound is considered as a wave.
Types of Waves
On the basis of direction of propagation, waves are categorized into two parts:
(i) Longitudinal waves: These are the waves in which the particles of the medium vibrate along the direction of propagation of the wave. For example: sound wave.
(ii) Transverse waves: In this type of wave the particles of the medium vibrate in a direction perpendicular to the direction of propagation of the wave. For example: waves produced in a stretched string.
Another type of waves which do not require any medium for propagation are called electromagnetic waves. These waves can travel through vacuum also. For example, light waves, X-rays.
Compressions and Rarefactions:
Compression is the part of wave in which particles of the medium are closer to one another forming the region of high pressure and density.
Compressions are represented by the upper portion of the curve called crest.
Rarefaction is a part of the wave where particles spread out to form a region of low pressure and density.
Rarefactions are represented by the lower portion of the curve called trough.
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