Scientists find out that there are three basic kinds of media for sound to travel through: solid, liquid and air. When sound waves move through solid, liquid or air, the molecules are vibrating in the matter. The molecules in solid are packed extremely tightly; the molecules in liquid are packed less tightly than in solid; the molecules in air are packed loosely. The reason is that solid has definite shape and volume; liquid has indefinite shape and definite volume; air has indefinite and indefinite volume. The speed of sound travelling in air is 340m/s, however, sound transmits four times faster in water than it does in air.

First, we focus on solid media such as wood, steel, plastic and so on. If a person is hitting a drum, the surface of drum will vibrate. Then he adds more force, and the sounds will become louder and clearer. If he strikes different parts of skin, the sound will also be different. People may wonder how to prove that the drum is vibrating and how to prove the loudness. We can put small pieces of paper on the drum surface. When the person strikes the drum softly, paper will jump. When the person strikes the drum hard, paper will jump higher.

Water is the most common liquid in the world and we can find that it is helpful everywhere. The particles in liquid are moving all the time so they can change their position easily. Whales can communicate in the sea over very long distances because sound travels faster in water than in air. Sound wave detector uses the same way to detect the image of sea below 5000 km in the sea.

Furthermore, human beings and terrestrial animals can hear sound from each other and communicate freely. Although air is not as efficient as liquid or solid, we are able hear sound in air and talk. The sound travels through a lot of materials in order for us to hear it. It transmits through air first and reach the eardrums. The eardrums vibrate and send the sound to our brains. We can understand the messages and respond them. The ability of human ears does not allow human beings to hear all the sound transmitting through air. Dogs, bats and dolphins can hear more sound than human beings.

In summary, sound cannot transmit unless there is no media in the whole environment such as in vacuum. The speed of travelling sound is different in different medias.

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