Occupational Health & Safety

A sound-level meter, as shown in this Casella USA photo, is a hand-held device that allows a competent third party to take measurements at the ear of a worker with the instrument pointing at the noise source.

The Use of Noise Dosimeters in the Workplace

Margin of error should be accounted for and the worst-case scenario measurement taken as the reading, particularly when close to an action level.

The industrial environment has changed drastically in recent decades with an increased level of automation within the workplace. This has given rise to many changes in employee work patterns. It used to be the case on the majority of production lines, an employee would stay in one place during his or her shift. Monitoring the worker's noise exposure with a traditional sound-level meter was the answer.

However, with the increase in completely automated production lines, employees now may supervise several machines. This means they move around from workstation to the next, varying their exposure to noise in a much more dramatic way than previously. This article examines how noise dosimeters play a more important role in these types of noise assessments.

Meter or Dosimeter
The only way to monitor precisely an individual's exposure to noise is by using either a sound-level meter or a dosimeter. A sound-level meter is a hand-held device that allows a competent third party to take measurements at the operator's ear with the instrument pointing at the noise source. By repeating this exercise for all operations an employee performs during the day, you can calculate his daily exposure.

Where it is difficult to get close to employees with a sound-level meter, as in the case of forklift truck drivers, or where workers are exposed to many different noise levels, they should wear a noise dosimeter. This is the case more often than not in the modern workplace, where if you are using a standard meter you would have to measure the noise levels at each location, find out how long the worker stays at that location, and then calculate an overall exposure. This can take hours of calculations to perform and will not always result in accurate measurements.

The Use of Noise Dosimeters
If mobile work patterns exist in your workplace and a noise dosimeter fits the bill, it is important to realize precisely how these instruments must be used and understand their limitations. Given the logarithmic nature of the decibel scale, a variance of only 1 or 2dB can often mean serious misinterpretation of noise levels. This margin of error should be accounted for and the worst-case scenario measurement taken as the reading, particularly when close to an action level.

A noise dosimeter consists of a microphone on a cable, which can be clipped to a collar. The microphone cable is then passed under the clothing to the unit itself, which is small enough to be located in a pocket or clipped to a belt. The dosimeter can then be started at the beginning of the shift. If it runs until the end of the working day, the noise dose can be directly read from the instrument or downloaded without the need for calculations. Another useful feature of noise dosimeters is that they will log the noise data so that when downloaded to a PC, the time history of the noise can be viewed. This gives the ability to analyze when and where high noise exposures occur. This can be even more useful when the dosimeter is placed on an employee who is prepared to make a list of the times and jobs he or she was performing throughout the day. This will give the employer the ability to see which operations most need noise control in order to reduce exposure.

A traditional noise dosimeter is fixed to the worker's belt, and then a microphone on a cable is attached to the collar near to the ear. You should make allowances for human nature. Employees fitted with dosimeters and their colleagues will often shout into the microphones, distorting the readings, so it's best to ignore the first few days' results until the novelty wears off.

One advantage of dosimeters is that if employees wear them for complete working shifts, the noise dose is measured in full. However, if you need to make several measurements of different employees in the same day, a dosimeter can be moved to different employees, as long as the measurements taken for each employee are representative of their working day. Most modern dosimeters also will project the noise dose forward to the standard eight hours, so no calculations are needed.

With innovations in digital technology, noise dosimeters are becoming smaller and smaller. The latest "badge" dosimeters have certain advantages over traditional dosimeters. Because the dosimeter is small and light enough to be worn on the shoulder, it means there are no cumbersome microphone cables. If there are no cables to get in the way, not only is it safer to wear, but also employees are less resistant to wearing it and much more likely to forget it is there. This means the quality of the noise data collected will be improved.

Because of the small size of badge-type products, it is also possible to mount them in more innovative ways, such as on a hard hat, close to the ear, without interfering with an employee's working process in any way. This allows the dosimeter to not be mounted on clothing at all, therefore completely removing it from the employee's mind.

Also, consider the use of windshields. They play a crucial role in any sound level measurement, even when indoors, to provide protection from dust settling on the microphone, as well as from knocks.

Standards and Accuracy
Noise dosimeters are manufactured to IEC 61252, the international standard for dosimeters. IEC 61252 "Type 2" regulations require a field calibrator of a dosimeter before each use. Field calibrators produce a noise signal, normally a tone of 1 KHz at 114 dB. It is best practice to run the calibration test after any period of field measurement, as well, to check that there has been no significant drift of the dosimeter during the measurement.

The dosimeter and the acoustic calibrator must be returned to the manufacturer for a full calibration every two years for a true calibration. A lab conducts a battery of tests, including testing measurements across all frequencies and levels to ensure the dosimeter still meets the requirements of IEC 61652.

The Post Process
Dosimeters measure all essential parameters for workplace noise regulations such as daily exposures and peak levels. However, to serve a purpose this data must be easily accessible and be presented in a format that is comprehensible to someone not familiar with acoustic terminology. Modern software will give the dosimeter user the ability to store data in a format that accounts for who was being measured, when these measurements were taken, and at what workstation. Software today can output data into reports automatically, including the average and peak time history, in a simplified format with all required data for workplace noise regulations.

Conclusion
Dosimeters are crucial in noise monitoring in today's modern working environment, with its highly mobile workers and varying noise exposure. Dosimeters provide valuable information by using logged time history data that show exactly when and where specific noise exposure took place. These details allow the implementation of proper controls to prevent hearing damage, which is the true end goal of any noise survey.

This article originally appeared in the September 2010 issue of Occupational Health & Safety.

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