Mining Publication: The Consequences of 'Leaky' Enclosures

From an engineering perspective, an ideal industrial noise control solution focuses directly on the actual source of the noise. Eliminating the noise-generating mechanism altogether obviates the need for other noise control treatments or hearing protection devices. However, in cases where attending the source is not feasible, engineering controls must be oriented toward blocking the path that the sound waves travel toward employees. Acoustical enclosures are commonly used as sound path treatments to contain the noise from a machine: alternatively, a control room/booth or equipment operator's cab may be used to isolate the worker from the noise. Anyone who has successfully used acoustical enclosures knows that the design, procurement, and installation process is deceptively simple. All too often first-time efforts fail to account for many of the constraints that can render the enclosure essentially useless. Things that cannot be overlooked include: providing convenient worker access (physical and visual); allowing for proper machine operation/product flow; and supplying fresh air or preventing undue heat or contaminants build-up inside the enclosure. All of these factors must be given careful consideration, otherwise the enclosure will not perform adequately. It is important to be aware if the drastic reduction in the amount of protection afforded by an earplug/earmuff if it is removed for even just a few minutes during an 8-hour work shift. The accompanying graph illustrates this effect for Hearing Protection Devices (HPDs) with four different Noise Reduction Ratings (NRRs). Assuming workers are exposed to a constant level of hazardous noise throughout the workday. There is a surprising drop in the effective NRR provided by any particular HPD after only 10 minutes of non-use. Further, if the protector is not used for 60 minutes per shift (i.e. it is worn for seven of eight hours) then the effective NRR if nearly all HPDs is reduces to nine decibels or less, regardless of the labeled NRR. Therefore, it is not sufficient to wear your hearing protection most of the time - you must wear it all of the time to adequately protect yourself from noise-induced hearing loss.