Compressor Casings Noise Reduction
Casings can be very efficient radiators of noise, particularly if, in an attempt to reduce cost, the thickness has been reduced to a bare minimum. The thicker the sections and the more inherent damping there is in the material the better are its acoustic properties. Cast iron is a very good material in this respect. It is not generally very successful to try the application of acoustic cladding to a casing after manufacture. It is likely to be expensive and may also act as a thermal insulator, raising the temperature of the compressor. Water cooling jackets act as a natural noise barrier and it may be better in some circumstances to use water, rather than air, cooling.
An acoustic enclosure is likely to be the most effective form of noise barrier. Practically any required attenuation can be achieved by enclosure. Twenty five dB is perfectly possible whereas techniques of noise reduction at source will only give about 5 dB. In designing an enclosure, reference can be made to the Handbook of Noise and Vibration Control. By far the best enclosure is one made of solid brickwork or some other heavy material. This is not practical for a compressor which needs to be moved, so the next best choice is a box made of heavy timber or steel partitions, lined with an absorbent material with no unsealed openings. It is the mass of the enclosure which is the main factor in suppressing the radiation of noise, but unless the interior is lined with an absorbent material, the level inside builds up with the result that the effect of the barrier is less. Cooling air should only be admitted through well-baffled ducts. The benefit of total enclosure can be easily vitiated by small openings for the passage of pipes.