Sound is a special kind of wave action which is usually transmitted through air in the form of pressure waves.
These waves are received by hearing apparatus of animals, including man, transformed into electrical impulses in the ear and carried to the brain which enables us to hear.Sound waves are generated in a number of ways, such as explosive expansion of gases, turbulent movement of liquids, vibrations of solid objects etc., which start a series of pressure waves in all directions.
The intensity of these waves diminishes as the distance from the object producing those increases. Sound waves are reflected and deflected by objects which happen to come in their way. Porous materials, such as perforated board, absorb these waves. When two or more than two such waves superimpose they reinforce each other and in opposite phase they may cancel each other out.