Class 9 Physics Sound Propagation of Sound

Propagation of Sound

• Sound waves move from the point of generation to the listener through the medium.
• When an object vibrates and produces sound, the particles of the medium at the point of generation begin to vibrate.
• This particle passes on the vibration to its adjacent/neighboring particle. The first particle comes back to its equilibrium position of rest. Then the second particles passes its vibration to the neighboring third particle and then again comes back to its position of rest. This process continues until the vibration reaches the listener and he is able to hear the sound.
• As we see, the actual particles of the medium don't travel from the point of generation to listener. It is only the vibration that travels through the medium. This is called Wave motion.
• A wave is a disturbance that moves through a medium when the particles of the medium set neighboring particles into motion. Since sound waves are characterized by the motion of particles in the medium, they are Mechanical Waves.

• Sound propagates in Compressions and Rarefactions.
• Compression : When a vibrating body moves forward, it pushes or compresses the air ahead of it thereby creating a region of high pressure and high density. These are called Compressions. They are represented by the upper portion of the wave curve.(C)
• Rarefactions: When a vibrating body moves backward, it releases or loosens the air ahead of it thereby creating a region of low pressure and low density. These are called Rarefactions. They are represented by the lower portion of the wave curve.(R)
• The peak of the wave is called Crest and valley of the wave is called Trough.
• Sound waves are longitudinal waves. The individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance. The particles do not move from one place to another but they simply oscillate back and forth about their position of rest thereby propagating only the disturbance from one place to another.

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