Active control works by destructive interference between the original sound or vibration field in the vehicle and that generated by a controllable, secondary, source. Physical limitations generally confine its usefulness to low frequencies and so that it complements conventional passive control methods. The development of powerful processors at an affordable cost and the increasing trend towards integration in vehicles has allowed the commercial implementation of active control systems by several manufacturers, mainly for the reduction of low frequency engine noise. As vehicles become lighter to achieve fuel efficiency targets, it is expected that active control will play an important part in maintaining an acceptable NVH environment, in terms of sound quality as well as overall level.
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