Pollution is anything that harms or disturbs the natural environment. Just as chemicals in a river or litter in the park are pollution, so unwanted noise can pollute too. Some people cause noise pollution by playing their radios too loud, but the main problems in many areas are aircraft and traffic noise. In science, a noise is a mixture of vibrations without any smooth, regular pattern, for example, the sound of an electric drill or a radio tuning from one station to another. Noise can be reduced by soundproofing. You have probably noticed the difference between the sound in a empty room and a furnished one. Some materials are very effective at  _ sounds. They are often made up of fibres or foam, because these trap layers or bubbles of air. Sounds do not travel well through air. Most of the curtains, carpets and upholstery in your home are good sound absorbers and will stop sound travelling through windows, walls and floors. A lot of noise pollution can be avoided if a machine is designed properly. Modern cars contain different materials to reduce the amount of noise coming from the engine and other moving parts, as well as noise produced by the car as it moves through the air. Have a look around next time you are in a car. What soundproofing can you see? By replacing metal parts in cars and other machines with parts made of plastics and foams, there is now far less noise than there used to be. Which of the following can be used in modern cars to reduce noise from engines?
Metals. Plastics. Radios. Chemicals.