Sounds are complex physical phenomena originating in the vibration from a source that propagates energy into a medium as an acoustic wave. Sounds happen continuously and are everywhere: there is no such thing as 'silence' on the planet. As physical phenomena, sounds are neither positive nor negative. They acquire meaning and produce an effect only when considered from the perspective of a listener. When sounds are unwanted, they become noise. When noises are too loud and persist too long, they become noise pollution.