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There are two approaches for the noise control problem: active noise control and passive noise control. Active noise control is a technique, which aims to cancel unwanted noise by introducing an additional, electronically generated, sound field. It is suitable for low frequency noise problems. An active noise control system consists of one or more control sources used to introduce a secondary disturbance into the structural or acoustic system. This disturbance suppresses the unwanted noise produced from one or more primary sources. An important property of this is that they are self tuning so that they adapt to small changes in the system being controlled. Passive noise control aims to attenuate unwanted sound by modifying the characteristics of the environment in which the sound source operates. To control passive noise, a barrier made up of material with high density can be put on the sound source. The aim of building a barrier is to redirect the acoustic power flow away from the source.
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Noise Dosimeters are used to measure the cumulative noise does in terms of a weighted sound energy over a period of normal working day for the purpose of assessing hearing damage risk. Noise dosimeters integrate the noise level near the ear of a person over an entire day and provide information about the total noise exposure in a single measure, thus similar to radiation dosimetry. Dosimeters are a form of sound level meter that have a DC output signal that is converted into a series of impulses, which are counted to provide a measure of the noise dose. Dosimeters are designed for individuals who are exposed to varying levels of high-intensity noise; thus they are small in size and simple to operate with a microphone that can clip on to the lapel of a coat of shirt. Dosimeters are capable of providing the user with information in the form of statistical parameter as well as the peak level over any predetermined increment of time and thus provide a time history of the noise exposure.
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Noise enclosures are used to control the noise radiated by industrial machines such as pumps and internal combustion engines. The passive transmission loss of enclosures at low frequency can yield negative insertion loss occurring at resonances of the enclosure panels backed by the internal air cavity. Active noise control provides a means to enhance low frequency performance by eliminating enclosure resonances. The proper design of a noise enclosure will account for operator and maintenance access, ventilation, safety codes, and possible needs for future expansion. The acoustic design of a noise enclosure is driven by the decibel reduction required to meet goals.
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