Page 237 - Advances In Productive, Safe, and Responsible Coal Mining

216 Advances in Productive, Safe, and Responsible Coal Mining

The most preferred approach is the first approach—controlling noise at the source.
However, this approach requires a thorough understanding of the noise-generating
mechanism, and usually involves modifying this generation mechanism. For example,
reducing the airflow speed from an aero-acoustically generated noise can result in sig-
nificant noise reduction.
When noise control at the source is not possible, then noise controls in the trans-
mission path should be considered. These controls usually have the form of acoustic
enclosures absorbing some of the noise and/or isolating the noise source from the
receiver (miner). It is very important that these enclosures are designed with heat
transfer considerations in mind to prevent overheating of noise-generating compo-
nents such as electric motors, gear boxes, etc. A drawback of acoustic enclosures
in underground environments is the reduced visibility that they can cause, thus creat-
ing safety concerns.
Finally, personal protective equipment (PPE) such as ear muffs and earplugs
should always be worn when noise levels are equal to or exceed 85dB(A).
A variety of styles and materials for hearing protection devices (HPDs) are commer-
cially available with features that allow them to adapt to being worn with other PPE
such as safety glasses and hard hats. Importantly, care should be exercised to guaran-
tee proper fit and consistent wearing of these devices.
All HPDs have a noise reduction rating (NRR) on the packaging, which provides an
estimated noise attenuation for that product when it is used correctly. However,
although they will protect the worker from hearing damage due to noise, compliance
credit is not given by the Mine Safety and Health Administration (MSHA) for their
use. In other words, MSHA’s rules for compliance do not permit subtraction of the
HPD’s stated NRR from the actual noise exposure level of the miner. Therefore,
although HPDs may in theory reduce the noise reaching the ear of the miner, the
employer is still responsible for reducing the noise exposure of that worker, as if HPDs
were not being used. This is one reason why engineered noise control solutions are still
needed despite the wide availability of HPDs.

12.3 Current technology

Advances in different areas of science and engineering provide specific tools that can
be used to identify noise sources, develop noise controls, and evaluate solutions both
in the laboratory and in underground environments. These advances in technology
result in the availability of various types of tools and materials that can be used for
purposes of noise control. This section presents some of these tools that have been
successfully implemented and used to develop noise controls in the mining industry.

12.3.1 Engineering software for acoustics modeling
Small- and medium-sized mining machines are usually tested in hemi-anechoic and/or
reverberation chambers to locate noise sources, and to determine sound power levels,
respectively. However, for larger machines, it can be difficult or even impossible to