Engineering Controls Database
Impact Trimming Machines – Noise Case Study
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Overview: The case history presented here is one of sixty-one case histories that were published by the National Institute for Occupational Safety and Health (NIOSH) in 1978 as part of an industrial noise control manual [NIOSH 1979]. The case histories are examples of engineering tasks that have been completed not only by professional noise control engineers but also by non-acoustical specialists who used common sense to solve their noise problems. The case histories were chosen primarily because the amount of noise reduction actually achieved was measured. Such engineering results, even if not directly applicable to a specific situation, illustrate general principles that may point the way to a successful result. They are intended to be useful to production and safety engineers, health personnel, and other factory personnel who are not specialists in noise control. Case study: Eight George Knight air impact trimming machines, located close together in a large production area, performed the trimming function once every 5 to 6 sec. An operator sat directly in front of each machine, and the sound level at each operator's station varied between 80 and 99 dBA. Since the eight workers were located in close proximity to each other, they received noise not only from their own machines (up to 97 dBA) but also from their neighbors' machines (up to 95 dBA). Equivalent daily exposures (time-averaged sound levels) for individual operators were found to be 91 to 92 dBA. |
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| Hearing loss is one of the most common occupational diseases in America today and the second most self-reported occupational illness or injury. Approximately 30 million workers are exposed to hazardous noise on the job and there are approximately 16 million Americans with noise-induced hearing loss. | |
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Examination of the situation revealed that the dominant portion of the noise exposure incurred by each operator was sound radiated directly to him from each machine. Since the amount of noise reduction required was small, it was clear that some redirecting of the machine-generated sound would be beneficial. The solution implemented consisted partly of partitions constructed around each workstation, as shown in Figure 1. The partitions were about 8 ft high and were made of 3/4-in. plywood covered on both sides with 1-in.-thick glass fiber boards faced with open-weave burlap. In addition to the partitions, see-through safety shields were placed between the contact point of each machine and the operator.  |
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| NIOSH [1979]. Industrial noise control manual – revised edition. Cincinnati, OH: U.S. Department of Health Education and Welfare, Public Health Service, Center for Disease Control, National Institute for Occupational Safety and Health, DHEW (NIOSH) Publication No. 79-117. | |
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air impact air impact fabric manufacture fabric manufacture trimming machine trimming machine |
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Measurements made subsequent to the final installation showed that the average sound level at operator stations was reduced from 91/92 dBA to 85/86 dBA. Maximum sound levels are now no more than 94 dBA. Figure 2 shows a statistical analysis of the present noise exposure. [img=2] |