Noise Control in the Workplace
By: Phillip Rauscher MPH, CIH, CSP, Contributor
When one thinks of an effective hearing conservation program, there are a few things that often come to mind. The first elements are an in-depth audiogram in which a skilled technician or medical professional provides feedback on results—good or bad—to help the employee make sound decisions when in noisy environments. You may think of a robust noise-sampling program where data is routinely used to make decisions to protect employees. You might even think of a great personal attenuation check to ensure that each employee has a hearing protection device that is well-fitted; gets them the required hearing protection factor; and is well-received. However, there is a time-tested element that is often overlooked in practice in otherwise robust programs: engineering controls.
There are several reasons we need to ensure that noise control is given its due in our hearing conservation programs (HCP). I am a firm believer that, when looking at the hierarchy of control, we all carry the professional pride to heed its warning; elimination, substitution and engineering controls should always reign supreme to ensure that employees are as protected as possible. By this point, we have all been told of the variability and reliability of person protective equipment (PPE), and hearing-protection devices are no exception to the rule. Earmuffs seals will wear out; foam ear plugs may not expand properly; and employees may simply forget to put flanged plugs in when distracted by work. This makes engineering controls the responsible choice.
If the morality of protection is lost on a seasoned employee or a particularly shrewd accountant, there is another undeniable benefit: lower cost. Some of the simplest fixes that will reduce noise also help to prolong the life of equipment or increase its productivity. Unlubricated bearings wear quickly; old belts slip; and improperly mounted equipment vibrates, causing stress. (More on some of these later.)
If this still doesn’t convince someone, remember, in the U.S., the OSHA regulation states that engineering controls need to be utilized when employees are exposed at or above the permissible exposure limit (PEL). Though there is a long-standing compliance memo that states the rule will not be enforced until 100dBA, this could change, given today’s tumultuous political landscape. Although it didn’t happen, a recent administration considered changing this long-standing memo and could do so with little warning.
Solving the Problem
In my time in the field, I got over the fear of engineering to help diagnose problem areas and came to realize not all problems need to be solved by a trained noise-control engineer (though if you have one lying in wait, that would be a great resource). The first step to determine the best place for noise-control effort was always to review employee sound exposure.
When looking facility-wide, personal exposure to employees, when measured, allows for the advantage to account for employee movements throughout the day. Though there may be a significant source of noise, sometimes these are naturally segregated from employees. For example, a pump room employees routinely avoid may be a significant source of noise but not a significant source of exposure.
When exposures to higher levels of noise are found in employee exposure, the next step is to do a walk-through of the employee work area. Like other qualitative assessments of the workplace, a noise- exposure assessment can start simply. Using our own hearing, we can likely determine a standout noise source simply by noticing that it is indeed very noisy surrounding that piece of equipment or, at the very least, a general area of concern in which the employee works.
If there are several sources that could be of concern, the next step is to start measuring. With a small amount of background knowledge, we can use a basic sound level meter (or a dosimeter with an instantaneous display, if that’s on hand) to help determine the characteristics of a particular problem. When set to the A-weighted decibel setting, a sound level meter will likely measure the same exposures that were collected by the dosimeter worn by the exposed employee during noise assessments.
The work area can be split into even-sized zones and noise levels; this will allow for a “map” of noise levels throughout the employee work area. Problem areas for noise generation will be of higher concern. Once a general area is identified, we can isolate a component of a complex machine in several ways.
The easiest way to determine which component of a machine is causing the concern is to turn systems on and off while other confounding noise sources are turned off. This could be after a shift or during lunch; a maintenance employee can help to turn on and off systems to help determine the source. This can isolate compressed air, gears, fans, bearings or belts causing noise.
If turning systems on and off isn’t an option, another option that requires more specialized equipment is utilization of an octave band analyzer. This could be integrated with a sound level meter or could be a “filter” that is attached to help separate the entirety of noise measurement into narrower “bands.” This can isolate high- and low-pitch noise.
Understanding the characteristic of the noise exposure can help to determine which component is likely the problem. If there is one octave that is particularly high, the sound level meter can be utilized to isolate this component. As we head into the future—smaller equipment can do more on a smaller platform—some personal noise dosimeters now have the option to measure octave bands, while simultaneously measuring A-weighted exposures required for compliance. This has the advantage of, rather than speculating the frequency of reoccurring noise exposures from throughout the shift, we can see these exposures all day. Once identified, there are several control measures that can be employed to lower overall exposure to employees.
Possible Solutions to Common Noise Sources
Often, one of the great drivers of any shop is compressed air. However, this is also a common source of noise. This is a case where more definitely isn’t better. I suggest examining the amount of air use for all functions in a facility. Reduced air usage comes at a reduced cost (compressor runs less often in addition to a lower noise-generation rate). If there are part ejection sites that use air, a modified nozzle or even just a lower flow rate that still does the job can help to reduce noise.
When purchasing nozzles, manufacturers can often tell you how much noise will be generated at the air flow rate you need to get the job done. Nozzles that reduce turbulence in air will also reduce the noise generated. When air flow is not needed, and instead is an exhaust from the process, exhaust silencers can be used that will greatly reduce noise generation.
Another common problem in the field is vibration of guarding: Every solution has a problem, right? Fortunately, there are several things that can help to alleviate some of the noise that is generated from guarding panels. If vibration is the major cause of noise from the panel, expanded metal can help to reduce the amount of noise generated by the panel.
If this is not possible, or if it does not reduce the noise level to the amount desired, we can reduce the amount of vibration in the panel with vibration-suppressing mounts. If a solid panel is utilized, adding mass to the panel in the form of stick-on rubber pads can help. Which side of the panel is treated will not matter in this case, as it will have equal effect no matter where applied. If the panel is part of a total enclosure of equipment, this could also double as a sound-attenuation device. Be sure gaps are closed and felt or rubber are used to minimize vibration between panels.
Every time a gear strikes another, it will cause an impact noise—increasing the number of teeth will reduce the force applied by each tooth and will reduce noise. Better yet, use a belt that has no impact noise from contact. Just keep the belts properly maintained, or else you will get slippage that will become a problem all by itself.
A minimal amount of math can be used to determine the frequency of noise generated by an axial fan. If we multiply the number of blades by the fan’s rotations per second, we will result in the frequency in hertz. If this is the predominant source of noise in the analysis, the simplest solution is to change out the fan. A new fan with different blade can change the frequency and noise generated, while still moving enough air to keep components cooled.
Centrifugal fans used in ventilation can also cause a problem. The amount of energy required to change the direction of air flow is one of the greatest challenges ventilation system fans are required to overcome. Every inefficacy in a ventilation system will create some kind of noise, but this is especially true when a duct turns too closely to the fan. In this case, proper use of a smooth duct with no turns at the inlet and exhaust of the fan will greatly reduce fan inefficiencies and noise.
With continued vigilance and sound programs, we can identify noise sources that contribute to the overall noise exposure of employees. Small measures taken daily by those in the field can help to eliminate noise-induced hearing loss as one of today’s most common occupational health conditions. These small measures do not require massive overhaul of facilities in most cases and, with a bit of thought and intention, can reduce equipment wear; reduce employee exposure; and become a vital component of a top-tier hearing conservation program.
Phillip Rauscher, MPH, CIH, CSP, is Senior Director-Science, Education & Publications, at ACGIH®.