Engineering Controls Database
Barley Mill – 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: Excessive sound levels existed around the Moorspeed and Ross barley mills (rolls 8 in. - diameter, 15 in. - long), a hay shredder, and a control operator's chair in a cattle feed grinding mill. The objective was to reduce the sound level at the operator's position for Occupational Safety and Health Administration (OSHA) compliance. |
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Sound level measurements at the control operator's chair, at the mills, and at the hay shredder were measured to be too high for 8 hours of exposure per NIOSH guidelines. Roller crushing actions produced high sound levels, and correction by machine redesign was believed to be too costly a method for solving this problem. When the source is too difficult or uneconomical to attempt to correct, working on the noise path will often result in a more economical solution. Therefore, a partial enclosure, open at the top, was chosen. 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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Walls can be of solid construction with access doors, because in this case access was needed for adjustment, maintenance, repair, and roll replacement. For roll replacement, a forklift truck entry was required. For ease of quick access, a fixed barrier wall was discarded in favor of a lead-vinyl curtain wall extending, if required, up to the 17 ft height of the roof support beams. All three noise sources could be enclosed by two curtain walls at the corner of the building, as shown in Figure 1. The curtains run on rails for easy sliding back and are held together by Velcro closures. For visual access, the enclosure can have 10 x 20 in. plastic windows placed to order; use only the minimal number. To reduce leaks, the curtains should be long enough to drag a bit on the floor. Some rerouting of power, steam, and air lines may be required. The preceding simplified treatment neglects an important fact: the noise has not been eliminated, merely redistributed. Thus, the total sound power from the machines escapes from the topless enclosure and spreads throughout the room. Close to the curtains, there should be some reduction, but very little farther away. Absorption is required for actual reduction of the sound powers. The enclosure developed by the curtain walls is, in effect, a separate small room, and the noise reduction can be estimated from the relationship of total absorption before and after adding the sound absorption panels. Resultant level = measured level - reduction = dBA. |
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Beranek LL [1971]. Noise and Vibration Control, McGraw-Hill, New York, N.Y. p. 178. 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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barley mill barley mill barriers barriers curtains curtains noise noise noise control noise control partial enclosures partial enclosures |
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The measured final sound level was 87 dBA. This level was 3 dB lower than the maximum desired sound level, and was the result of paying careful attention to elimination of leaks. The room formed by the curtain did not realize such a reduction, but since these machines required no attention while running, the noise exposure of personnel was significantly reduced below original levels. The major remaining path is reflection from the ceiling. Comments: Barrier walls of various heights can often be used between a noise source and a machine operator. A major pitfall is that, in a room with a high level of reverberant noise, the partial barrier will be short-circuited by the reflected noises from walls, ceilings, and other surfaces. In such cases, attenuation based on the partial wall theory may not be obtained, and the result may often be no attenuation at all in highly reverberant rooms. Curtain walls must be kept closed to get attenuation. Sound-absorbing units must be kept clean to be efficient. Even in a semi-reverberant room, a reduced barrier height can be used. In this case, a 7 ft barrier should ideally reduce the level to 89 dBA at the receiving location. However, since the semi-reverberant conditions will introduce more reflected sound with the lower barrier, the high wall used in this case history is recommended because the added absorption within the barrier area has, in effect, made a separate small room and created the condition on which the barrier wall theory was based. |