Engineering noise controls are the preferred solution to a noise problem because they address noise sources directly. Administrative controls and personal protective equipment should be explored as secondary solutions. Basic noise controls include barriers and sound-absorbing materials. A barrier is a solid obstacle that is somewhat impervious to sound and that interrupts the direct path from the sound source to the receiver. For the best reduction in sound level, the barrier should be: 1. placed as close as possible to either the source or receiver; 2. assembled to be as tall and wide as practical so it extends well beyond the direct source-receiver path; and 3. constructed of a material that is solid and airtight. Sound-absorbing treatments reduce reflections and the resulting echoes and reverberation. Usually, these materials are porous. Compared to high frequency sounds, low frequency sounds are more difficult to absorb with materials and to block with barriers. Therefore, it is important to know the frequency content for a particular noise problem. The effectiveness of barriers and absorptive materials as noise controls on mining equipment was tested during field studies. Following are some of the key findings. 6.1 Haul Trucks: The use of absorptive materials in the operator's area of tested haul trucks had very little effect on sound levels underground. Sound level reductions were on the order of 1 dB(A). Most of the sound reaches the operator via the direct path from the noise source to the operator. In addition, noise reflects from the walls to the operator station. Open cabs allow the direct and reflected sound to enter the operator station. Therefore, a large reduction in sound levels from installing sound-absorbing material at the operator station is not expected. 6.2 Load-Haul-Dumps (LHDs): A fully enclosed environmental cab can provide 20 dB(A) or more of noise reduction. If a fully enclosed cab is impractical, a partial cab can provide useful protection as long as the openings face away from the primary noise sources. A partial cab with three sides and a top was found to provide more than 10 dB(A) of noise reduction. Both full and partial cabs should have similar results on other underground equipment. When installing a retrofit cab, it is wise to contact the original equipment manufacturer to ensure that the integrity of the falling object protective structure (FOPS) is not compromised. 6.3 Jumbo Drills and Bolters: When applying noise control treatments, care should be taken to use the right product for the job. The 0.5-inch-thick rubber conveyor belt mats used to cover the electric-motor-powered hydraulic pumps on the jumbo drills and bolters were effective at reducing noise because the heavy rubber is a barrier material, which is the correct choice for the application. Rubber is usually not the best material to use for a barrier, but in this case it was effective. On bolter 2, the electric motor and hydraulic pumps were covered with sound-absorbing material. In this instance, the treatment had almost no effect on the noise from the electric motor and hydraulic pumps because sound-absorbing material makes a poor barrier. Sound-absorbing material is most effective when it is used at a reflective surface. The cover should have been constructed using a barrier lined with sound-absorbing material to surround the electric motor and hydraulic pumps. Prior to developing noise controls for a source, the significance of this source should be considered relative to other noise sources on a machine. In this case, the sound level with the electric motor and hydraulics operating was 85 dB(A) whereas noise due to drilling and bolting is about 100 dB(A). In this case, the noise due to the electric motor and hydraulic pumps is insignificant. Windshields on jumbo drills and bolters reduced the sound level at the operator's station during the drilling/bolting cycle up to 3 dB(A). The noise generated from drilling and bolting is relatively high frequency in nature. Therefore, the windshield provides an effective barrier. Gaps between and around sections of the windshield should be sealed for the most effective noise control. In addition, wrapping a windshield around the operator station improves the noise reduction by forcing drilling/bolting noise to travel further around the windshield to get to the operator. In addition, this helps block noise reflected from the rib from reaching the operator. 6.4 Lessons Learned: Through evaluating different noise controls on underground machinery, NIOSH researchers discovered several findings. Both the effective and ineffective treatments rendered valuable information. 1. Although it is tempting to use sound-absorbing materials for noise controls because they are inexpensive and simple to attach to existing surfaces, sound barriers were always more effective in the examples NIOSH studied for this report. 2. Windshields and environmental cabs can be highly effective noise controls, especially for high frequency noise. 3. Plugging gaps in machine panels and windshields with a material that creates an airtight seal can greatly enhance the noise reduction benefits of existing barriers. 4. Gaps in barriers compromise noise control effectiveness. 5. When openings in enclosures are necessary, a partial enclosure can provide some benefit. Enclosures should be lined with an absorptive material thick enough to absorb the dominant sound frequencies. Openings to let air in and out of the enclosure should have lined ducts with multiple bends to absorb sound and to force it to follow a circuitous pathway before exiting the enclosure. In order to reduce noise-induced hearing loss from work-related noise exposure, mine workers, union representatives, mine managers, equipment manufacturers, NIOSH, and MSHA must work in partnership to successfully construct and implement new and better noise controls. To ensure the success of a noise control program, appropriate materials must be applied and the noise sources treated must be significant in terms of the worker's daily noise exposure.
Noise-control; Noise-absorption; Noise-sources; Sound-attenuation; Engineering-controls; Control-technology; Personal-protective-equipment; Personal-protection; Protective-equipment; Noise-frequencies; Mining-industry; Mining-equipment; Underground-mining; Noise-induced-hearing-loss; Hearing-conservation; Hearing-loss; Hearing-protection; Metal-mining