Published in the May 2005 American Longwall Magazine

Work-related hearing loss remains a critical workplace safety and health issue. Industry studies have shown an estimated 90% of US coal miners will have some hearing loss by the age of 52.

Introducing engineering controls to remove hazardous noise is the most effective way to prevent hearing loss, as David Yantek of NIOSH’s (National Institute for Occupational Safety and Health) hearing loss prevention program explained, but it is almost impossible to find a generic solution for noise reduction.

Noise exposure can be controlled by introducing noise controls, by using administrative controls, or, as a last resort, by using hearing protection.

“Developing engineering noise controls is the best solution since noise is reduced at the source. With lower sound levels, workplace communication improves and warning sounds can be better heard,” Yantek said.

The Coal Noise Partnership, which includes the Bituminous Coal Operators Association (BCOA), National Mining Association (NMA), and the United Mine Workers of America (UMWA) along with Mine Safety and Health Administration (MSHA) and NIOSH, provides input to guide NIOSH’s efforts for developing engineering noise controls. In addition, NIOSH and MSHA noise exposure data is used to determine where engineering noise controls are needed most.

MSHA data indicates continuous miners and roof bolting machines are the main sources of noise overexposure in underground coalmines. (In surface and underground coalmines, continuous miners are responsible for the highest number of overexposures, bulldozers are next highest, and roof bolting machines third).

Yantek said on a continuous miner, noise is typically generated by the conveyor chain, cutter head, dust scrubber, electrical motors and pump.

Tests of a continuous miner NIOSH keeps at its research site for experimental work identified the conveyor, cutting head and scrubber as the major noise contributors. In some cases, the conveyor is the most significant noise source. This does not mean, however, that the problem will be identical at all mines or with all continuous miners, Yantek warned.

Last year NIOSH successfully completed a project targeting reducing noise from continuous miner conveyors. A polyurethane coating was applied to the flight bars of a Joy continuous miner conveyor.

The coated flights reduced the sound power level generated by the conveyor by 7-8dB(A) as measured in NIOSH’s reverberation chamber. In addition, measurements in an underground coalmine showed the sound levels near the tail section of the continuous miner were reduced by 4-6dB(A) when using the coated flights. Tests at a mine indicated the coating was durable, with little wear in five months under operating conditions. Further trials are currently underway.

The most recent development in hearing loss prevention work is the start up of a pilot study to determine the significance of continuous miner cutting noise.

Using a combination of measurements and past research efforts, Yantek suspects cutting noise contributes about the same to noise exposure as conveyor noise.

“It’s a tough situation as far as developing noise controls goes because now you’re talking about multiple noise sources,” Yantek said. “The engineering noise control approach is sometimes difficult because you may not have a single noise source that is dominant and, if two sources are equivalent, any reduction in one of them may not deliver a major result.”

This is because sound accumulates in a logarithmic, as opposed to linear, fashion. This means if two noise sources each generate 90dB(A) by themselves, together they generate 93dB(A), not 180dB(A). Therefore, eliminating half of the noise generated by a piece of machinery would only reduce the sound level by 3dB(A).

So the first step in developing engineering noise controls is to determine all the noise sources on a machine and develop tests to indicate where they rank.

Making measurements of cutting noise by itself is difficult – the operator can’t really cut coal for an extended period of time without running the conveyor, or he’d bury the machine in coal.

Yantek is trying to find a way to work with a mining company to make measurements of cutting noise in 15-second increments with conveyors turned off. It’s about the best he can do under the circumstances.

Depending on the outcome, if cutting noise is shown to be less significant in the overall noise level, research efforts may return to the conveyor chain. Various permutations of coating different parts of the conveying system have been tried, including coating the tail roller, the bottom return deck, and the conveyor chain itself.

Coating the tail roller reduced sound levels but the coating used proved unable to withstand the rigors of the environment. NIOSH is currently working on a composite tail roller that uses a steel sleeve to protect the coating on the tail roller. With the protective sleeve, the polyurethane coating may still yield a reduction in radiated noise and the coating would be protected from the harsh conditions. Yantek believes further reductions in conveyor noise can be made by returning to the tail section in more detail.

For the moment, though, cutting noise is the focal point of research on continuous miner noise.