At the highest levels, noise can induce hearing damage. In most areas, automobile and truck traffic is the major sources of environmental noise. Traffic activity generally produces an average sound level that remains fairly constant with time. Air and rail traffic and commercial and industrial activities are also major sources of noise in some areas. Passive control is the traditional method of creating a physical barrier barring the penetration of the waves.
Passive techniques work best on medium to high frequency noise.
Lower frequency noises have more energy and thus more material is needed to absorb the energy rendering passive noise control methods bulky. Active control essentially tries to eliminate sound or vibration components by adding the exact opposite sound or vibration waves. Two waves with equal amplitude and opposite phase add together destructively, resulting in a canceling of overall amplitude. The phase describes the relative position of the wave in its rising and falling cycle. If two waves are in phase, they rise and fall together, whilst if they are exactly out of phase, one is rises as the other falls, and so they cancel out.