tuning fork #1

tuning fork #2

tuning fork #3

   
A sound wave moves through matter, such as air, just like a wave moves through a slinky.The pictures of a tuning fork at three stages help you visualize how air molecules might look around a tuning fork.

Look at tuning fork #1. When the tuning fork is at rest, the fork is surrounded by molecules in the air.

As a tuning fork's prongs move apart because of a vibration, the molecules ahead of it are crowded together. (See tuning fork #2). They look like they are being pushed together. They bump each other.

As a tuning fork's prongs come back together, (see tuning fork #3) it leaves a region that has fewer molecules than usual.
The region of a sound wave in which the molecules are crowded together is a compression. The region of a sound wave in which particles are spread apart is a rarefaction.

As a tuning fork vibrates, it causes molecules in the air to move. The molecules bump into other molecules nearby, causing them to move. This process continues from molecule to molecule. The result is a series of compressions and rarefactions that make up sound waves.
Unlike light waves, sound waves do not travel through a vacuum. They need matter to travel. That is why sound can travel through a wall.

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Naga Ramesh
IIT Kgp - 2005 batch