# Engineering Controls Database

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## Speed Control Device – Noise Case Study

| Problem:                         | 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: The speed control for a rapid transit system controls the train speed by electronically varying the voltage delivered to the traction motor. Basically, the speed control (illustrated in Figure 1) consists of a main box that houses the electronic components: a scrubber blower that is used as part of the air cleaning system, and a fan-cooled motor that is used to drive the main blower.[img=1] |
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| What this means to you:          | 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.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
| Description of control:          | In general, the operational noise of the speed control is predominantly the aerodynamic noise caused by the fans, determined by measuring the noise with the fans inoperative. Noise control considerations included source redesign, since the noise-making equipment was made by the investigators on this project.  The major noise reduction was accomplished by (1) using a smaller main blower impeller since, for a given speed, smaller impellers are quieter than larger ones, (2) removing the scrubber blower and using a static air cleaning device, and (3) installing a specially designed muffler at the main blower inlet. Figure 2 shows the essential features of the treatments.**Figure 2. Essential features of a muffler.**                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
| References:                      | 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.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
| NAICS Codes:                     | 485112                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
| Keywords:                        | noise controlnoise controlrapid transitrapid transitspeed control systemspeed control system                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
| How well does this control work? | The cumulative effect on the noise is shown in the polar plot of Figure 3.  An average sound level reduction of 7 dB was achieved.[img=3]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |

November 16, 2018
National Institute for Occupational Safety and Health