If sound is annoying or loud enough to damage hearing, it is called noise. Noise has many effects on people . In addition , hearing loss, and lowers performance in mental and physical tasks. Conventionally, noise reduction has been implemented with vibration isolators , acoustical absorbing materials , enclosures , barriers , and vibration damping materials. Generally , these approaches to the noise control are a relatively inexpensive way to reduce noise. They are called passive noise control methods because they do not need power to operate . At low frequencies , the noise reduction levels achieved are often inadequate and control of low frequency noise with passive methods only is often impractical as well as costly. For example, mufflers must be large and absorbing walls heavy to reduce noise levels effectively. Fortunately, active noise control can complement passive noise control . The idea behind active noise cancellation is to generate opposite phase sound that destructively interferes with the existing sound field. Active methods are more costeffective and capable of adapting noise levels at low frequencies, but at higher frequencies of, say, over 500 Hz, complex sound fields and the larger computational load needed in a control system due to higher sampling rates limits system performance greatly. Comprehensive noise control solutions often combine both of these approaches. Active noise control is a combination of acoustics, signal processing, and mechanics. Although the principles behind active noise control are rather well understood, one has to have a detailed understanding of the limitations and possibilities of those areas to be able to produce a practical application for active noise control. The concept of ANC was patented for the first time in the 1930s, but not until the 1990s did practical implementations became available. In the 1990s, control algorithms and the computing power of digital signal processors were developed to a sufficient level to make practical control systems possible. Nowadays there are not many industrial applications using active noise control, but practical systems can be found, for example, in active headsets, airconditioning ducts, and propeller aircraft.
Keywords : Active Noise Control, simulation model, modelling, control system.
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