Page 38 - OHS, March 2020

HEARING PROTECTION
Developments in Personal Noise Dosimetry
Workplace noise measurements are critical to keeping workers and workplaces safe; make sure your noise dosimeters are appropriate and up to snuff.
BY BOB SELWYN
Workplace noise measurements are a critical aspect for today’s industrial hygienist and health and safety profes- sionals. High levels of noise still con- tinue to exist in many manufacturing establishments in the industrialized world. While overseas countries have been using a combination of hand-held sound level meters and a smaller number of personal noise dosimeters, the preference in North America is to es- tablish a worker’s noise dose using a personal device.
The advantages of a personal noise dosimeter are many for the busy professional who may have many other hazards to deal with in the workplace. First, today’s designs of noise dosimeters are small, usually about 3-by-12 inches and lightweight in the order of three or four ounces. Gone are the days of the tradi- tionally worn units that could be as large as 6-by-3 inches and weigh in at around 14 ounces, hanging off the user’s belt with a remote cable microphone.
Current noise dosimeters typically feature a mi- crophone that is built in to the body of the instrument and worn on the shoulder in the hearing zone close to the ear. This has the added advantage of much smaller size that is less likely to get in the way when workers are moving around in confined spaces, and it elimi- nates one of the major weaknesses of the belt design: the microphone cable connections.
Users report being much happier wearing the smaller instruments, and they become used to them so that after a short while, they may even forget that they are wearing a noise instrument. The ability for the instruments to go where the worker goes is important for one main reason: it means the IH professional does not have to worry about using a hand-held instrument and following the worker throughout the day, which increases the likelihood of the IH professional missing certain contributions to the daily exposure.
The sensor is the most sensitive part of any sound measuring instrument, and microphones on noise dosimeters are no exceptions. They have traditionally been of the pre-polarized electret design featuring a thin diaphragm that responds to the changing noise levels. To maintain the required accuracy, the dia- phragm is a very thin membrane that ages and can go out of tolerance after a few years of hard usage. The only remedy for this is to replace the whole micro- phone capsule for a new one to maintain the calibra- tion ability of the whole instrument.
Some new dosimeters now feature solid state mi- crophones using the MEMS technique (Micro Elec- tro Mechanical Systems) to offer an extremely robust
design that is virtually user-proof and aging-proof on the instrument. The obvious benefit of this design is the longer life of such MEMS microphones, with some suppliers offering lifetime warranties for the mi- crophone for as long as the user has the instrument in use. This is a significant financial advantage for the end user who will not be faced with replacement costs every four or five years when returning the dosimeter to be recalibrated each year.
The traditional benefits of the hand-held sound level meter were that the IH could see and hear what was going on and make written notes about the poten- tial sources of high noise levels during the day. This is both costly to carry out for a whole shift and prone to potential errors if you fail to notice the occurance.
A more secure and reliable methodology in many current dosimeter designs is the optional ability to set a threshold sound level such that the instrument will record the actual audio arriving at the microphone. This is saved in a format that allows the recorded sounds to be played back after download to the soft- ware that is supplied with the instrument kits. Once saved, the IH can listen to the sound any time they want to hear exactly what the source was at that par- ticular time. It becomes much easier to distinguish between a turbine, a compressor or a drill simply by listening to the recording to know what was happen- ing during the worker’s shift.
Audio recordings can be made at good enough resolution and quality to be able to identify the noise sources, and it is even possible in some dosimeters to record the whole of the day’s shift if required. Thresh- old triggered recordings help to limit the actual re- cording time to more practical limits, such as only above 95 dBA for example.
The other key advantage of the hand-held sound level meter was the ability to perform octave band fre- quency analysis of the noise to help select the most appropriate noise control methods to reduce the high levels, or at least to assist in the correct prescription of the best hearing protectors as a short-term solution.
Using the hand-held sound level meter usually involved going back to the worker after downloading results and analyzing the noise, then trying to return to the same place at the same time in the day and discover what was producing the loud noise. Some newer shoulder-mounted noise dosimeters feature optional octave band analysis all the time during the recording of the noise exposure results. This has the extra advantage of not needing to go back and repeat measurements when it may not be possible to know
32 Occupational Health & Safety | MARCH 2020
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