Ducts noise control

Few would dispute that engineering noise controls involve a high level of expertise. Mismatched vibration isolators could set up a sympathetic reaction and bounce a machine off its mounts. Noise absorbing coefficients of sound insulation have to be matched to the spectrum of the noise. Noise treatment of duct work has to take into account required air flow. But what about other measures like testing hearing and issuing earplugs ... 

The use of active noise control should be considered when passive noise control methods are expensive or difficult to install. The control of very-low-frequency noise in a duct is an example where passive noise control may be difficult. In low-frequency applications where space is limited and no additional pressure is available, active noise control becomes even more attractive. However, it is important to recognize that the microphones, electronic circuits, and loudspeakers will require at least some ongoing maintenance, particularly when installed in hot or corrosive environments. 

For special applications requiring significant attenuation of low-, mid-, and high-frequency noise, it might be most practical to consider the use of both active and passive noise control methods. A passive noise control treatment, such as a small muffler or duct lining, can be installed to attenuate the mid- and high-frequency noise and active noise control might be employed to attenuate the low-frequency noise. 

The car, which he runs on a daily basis, is a restored, eight-cylinder 1978 Chevy Camaro with stock 350 (5.7 litre) engine, no computer controls, automatic transmission, stock 4-barrel carburettor and stock fuel pump. The fuel tank has been replaced with a metal water tank with the filler cap vented to release heat and pressure. The exhaust was replaced with a new 2 inch pipe which is ducted into the water tank. The water tank has baffles inside it which also muffles the exhaust noise. The stock exhaust manifolds were used, but they will rust on the inside - custom stainless steel pipes would be best but these were not used due to their cost. 

Sound-absorption data on various materials are presented here in terms of the acoustic properties of rooms these data are also valuable for other applications, such as the acoustic treatments for ducts and tunnels. Architectural applications include the control of noise level in offices, stores, restaurants, and other occupied spaces, and the control of reverberation time and elimination of echoes in concert and lecture halls. 

Air-conditioning ducts in homes and offices often had asbestos coverings for noise and fire control. Phosphate fiber paper should have been outstanding in these applications. 

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