Measuring noise exposure
Three keys to achieve accurate readings
We all know that the requirements for OSHA workplace noise measurements involve establishing the individual noise exposure for our workers to be sure that we are protecting them from excessively high noise levels. High noise levels over prolonged periods of time can lead to premature deafness, lack of concentration, unsafe stress on the job and poorer quality manufacturing. Working in lower noise levels decreases the risks of workers’ hearing loss and results in improved product quality and reduced stress in the workforce.
Overall noise exposure level
The overall noise exposure is expressed as the Time Weighted Average noise level (TWA). This is intended to include all significant noise events during the day and express the result as a single overall number in decibels. The OSHA regulations, CFR 1910:95, require the measurement of the TWA in two slightly different ways depending on whether we want the Permitted Exposure Level (PEL) or the Hearing Conservation Amendment (HCA) result.
For the original requirement, which is called the PEL result, the noise levels must be assessed with a cutoff level of 90 dB. This limit is used to exclude any noise levels of less than this value. For the HCA result, we need to include everything that occurs above a cutoff noise level of 80 dB.
During the calculation of the TWA results the noise level is measured with the A frequency weighting, the Slow time response and a 5 dB exchange rate. This last parameter is sometimes overlooked but is very important. It represents the degree of “risk” associated with different noise levels based on a reference level.
If we consider the reference level of noise during the day to be equivalent to a continuous level of 90 dB for an eight-hour shift, then twice the risk would be encountered for a noise of 95 dB also lasting for the full eight-hour shift. Each time the noise level increases by 5 dB for the same time interval the risk of hearing damage is considered to be twice as risky. So the exchange rate of 5 dB defines our risk due to increasingly higher noise levels during the shift.
In practice, the noise levels at work are not usually steady for the whole of the day, so the noise is sampled and averaged over time to arrive at a single number time average level or LAV. The LAV is measured during the shift that may last less than eight hours or may be more than eight hours for workers who have longer shifts. In all cases, the TWA is calculated to express the total noise accumulated during the shift to an answer that represents an exact eight-hour working day.
Single very high level impact noise levels
The second requirement is to be sure that no single noisy event exceeds a peak level of 140 dB. This is to limit the exposure from any one very high noise impulse, which can also be harmful.
Based on the above explanation it can be seen that for every increase of 5 dB in noise level the duration must be decreased by a half to stay at the same level of overall exposure risk based on the OSHA exchange rate.
Once a noise level of 115 dB has been reached the allowable duration is down to only 15 minutes.
High noise level impulses then must be kept under 140 to prevent all of the days’ worth of harmful noise energy being condensed into a single high peak.
For the peak measurement, the frequency weighting in the instrument must be set to an unweighted or zero weighting and the A curve correction must not be used -- otherwise the low frequency content will be severely attenuated and not taken into account properly. In modern instruments we use the Z weighting (for zero response) to measure the peak level and this is independent of the Slow or Fast time response to be sure of capturing the true maximum peak level in dB.
Reducing noise exposure
The third requirement is to protect workers’ hearing when high noise levels exist. This is accomplished by reducing harmful noise reaching their ears. For many employers the noise levels will often be found to exceed the TWA limits, so there is a requirement to reduce the exposure for the affected worker(s). The best solution is to reduce the noise level at its source if at all possible, since this will have the optimum effect on the workforce. Where it is not practical to use quieter processes or machinery, the alternative approach is to reduce the noise levels actually entering the workers’ ear canals.
This involves the use of personal protective equipment (PPE) which in the case of risks to high noise exposure means ear muffs or ear plugs. In both cases the PPE selected must be capable of reducing a worker’s exposure below the HCA level of 85 dB for the eight-hour shift. Different PPE solutions offer different performance characteristics based on their inherent design. The performance of hearing protectors must be documented for every product that is available to enable the employer to choose the most effective for the specific noise problem.
The performance of different PPE devices varies with frequency; some offer better low frequency reduction and others better mid- and high-frequency reduction. A full understanding of the frequency characteristics of the noisy problem is best to make the right choice of suitable protectors.
Let us look behind these requirements in more detail and see how these are actually accomplished in practice.
More detailed time history results: To best understand the way that an individual’s noise exposure of TWA or the actual noise dose is made up, a detailed time history is very useful to show when the noisy times of the day actually occur. This allows the necessary control or protection to be most effectively used. Breaking the whole work shift down into say one second increments and saving the changing noise levels like this during the day will give the employer the best guide to what is going on and what contribution it has on the overall TWA result.
In some cases where there are many very short duration impulses contained within the overall exposure of a worker, even faster sampling will be advantageous. Samples as short as every 1/10 of a second will serve to define the true detailed time history experienced during the shift.
Potential data corruption acquired during measurements: For correct identification of the peak noise limit there may be occasions when a mechanical bump or knock to the instrument causes a false result that is not truly representative of the actual acoustic noise level heard. It is simply something rubbing against the microphone or the dosimeter being tapped accidentally (or deliberately) by the worker. Identifying which of the samples are real noises can be achieved in newer noise dosimeters by the inclusion of a self-vibration sensing device that “tags” the measured time history results with a marker if a certain level was captured when a bump was also detected. This allows the employer to easily see and remove artificially corrupted results due to vibration and not the noise experienced by the worker.
Collecting the right information to select PPE: The performance of hearing protection devices is very frequency dependent. It makes sense to have the best information to help choose the best solution. The single overall A weighted reduction figure may hide the fine detail to know if PPE model X is going to be more effective than model Y. A dual measurement of the C and A weighted noise exposure levels, or more importantly the difference between the LC and the LA values during the run, quickly point to the likelihood of any low frequency contribution because of the differences in the shapes of the C and A frequency characteristics. A larger value of the LC-LA, or L(C-A), result indicates more low frequency content, which will point toward a different PPE product than if the value is smaller.
The three secrets for better personal noise exposure measurements are: 1) good resolution time history results to better than one sample per second; 2)self-vibration detection to prevent accidental or deliberate corruption of noise samples during the day; and 3) simultaneous collection of full frequency band results in addition to the overall noise levels.
Newer shoulder-mounted noise dosimeters now offer all these significant advantages in small lightweight dosimeters that are quick to setup, take all the results that are necessary, and produce easy to use report generators for customizable hard-copy outputs.
This article was provided by Sensidyne® Health and Safety Instrumentation, www.sensidyne.com.