One of the most familiar forms of energy in daily life is sound. We hear natural sounds like birdsong and wind. We hear the noise of vehicles and machines, and sounds such as speech and music from radios, televisions and stereo systems. We also rely on sounds to communicate when we talk to others.

Sounds are made by objects that vibrate (move to and fro rapidly). As an object vibrates, it alternately pushes and pulls at the air around it. The air is squashed and stretched as the molecules of the gases in air are pressed close together and then pulled farther apart. These are regions of high and low air pressure. They pass outwards away from the object in all directions. They are called sound waves.

Sound waves start as the energy of movement in the vibrations. This is transferred to the energy of movement in air molecules. As the sound waves spread out they widen and disperse, like the ripples on a pond after a stone is thrown in. So the sound gradually gets weaker and fades away. However if there is a hard, smooth surface in the way, such as a wall, then some sound waves bounce off it and come back again. The bouncing is known as reflection and we hear the returning sound as an echo.

Sounds also travel as vibrations through liquids, such as water, and solids, such as metals. The atoms or molecules are closer together in liquids than in air, and even closer still in solids. So sounds travel through them much faster.

            An object that vibrates to produce sound waves is a sound source. A bow rubs over the cello’s string and makes it vibrate. The vibrations pass into the air and also to the cello’s hollow body making the sound louder and richer.

The speed of sound varies depending on the substance it travels through. Atoms in steel are closer than molecules in air, so the vibrations of sound move faster and further.

Picture Credit : Google