Reducing impact noise from cocoa beans
Many process machines radiate noise due to multiple impacts from the components being handled. In some cases noise levels can be reduced by lining hoppers, chutes and conveyors, but in others the ambient conditions do not allow such techniques. Alternative approaches then have to be considered.
One such example was a machine used in processing cocoa beans. The beans were transported along a vibrating tray beneath an array of gas burners. Impacts between the hard bean cases and the steel conveying tray produced A-weighted noise levels of between 88 and 90 dB. Resonance in the cavity between the vibrating bean tray and the bed of the machine was also a contributory factor.
The conventional remedy of cushioning impacts from the beans by providing a lining of rubber or similar material for the bean tray could not be used here because of the high temperatures.
It was decided that the most effective way of reducing the amount of vibratory energy reaching the external radiating surface of the machine would be to reduce the response of the structure at the point of impact, ie at the bean tray itself.
This was achieved by applying point-fixed damping to the tray in the form of 2 mm thick steel plate, plug-welded to the underside of the bean tray between its existing tubular steel stiffeners. Acoustic resonance in the void beneath the tray was reduced by filling it with 96 to 128 kg/m3mineral wool slab.
About £2000. (1995)
A noise reduction of 10 dB.
Information supplied by Sound Research Laboratories Limited, Sudbury. Photographs courtesy of Micronizing Company (UK) Ltd.