SOUND
Sources of sound all have some part which vibrates. A guitar has strings, a drum has a stretched skin and the human voice has vocal cords.The sound travels through the air to our ears and we hear it. Evidently soound cannot travel in a vacuum as light can. Other materials, including solids and liquids, transmit sound.
Sound waves are reflected well from hard, flat surfaces such as walls or cliffs and obey the same laws of reflection as light. The reflected sounds forms an echo.
The speed of sound depends on the matrial through which it is passing. It is greater in solids than in liquids or gases because the molecules in a solid are closer together than in a liquid or a gas.
In the air the speed increases with temperature and at high altitudes, where the temperature is lower, it is less than at sea-level.