Page 48 - Occupational Health & Safety, October 2018

HEARING PROTECTION
Recent Developments in Personal Noise Dosimetry
BY BOB SELWYN
worry about missing certain contributions to the daily exposure, as might happen if using a hand-held instru- ment and following the worker throughout the day.
The sensor is the most sensitive part of any sound measuring instrument, and microphones on noise dosimeters are no exceptions. They have tradition- ally been of the pre-polarized electret design, featur- ing a thin diaphragm that responds to the changing noise levels. To maintain the required accuracy the diaphragm 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 electro mechanical systems) to offer an extremely robust de- sign 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 micro- phone 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 the IH fails to notice what was occurring.
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 micro- phone. This is saved in a format that allows the re- corded sounds to be played back after download to the software that is supplied with the instrument kits. Once saved, the IH can listen to the sound any time he wants to hear exactly what the source was at that particular 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 that used to be the do-
Workplace noise measurements are a critical aspect for today’s industrial hygienist and health and safety profes- sional. 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 lots of other hazards to deal with in the workplace. First, today’s designs of noise dosimeter are small, usually about 3 in by 2 inches, and lightweight, in the order of 3 or 4 oounces. Gone are the days of the traditionally worn units that could be as large as 6 in 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, that of the microphone cable connections.
Users report being much happier wearing the smaller instruments and soon become used to them, such 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 not to be overlooked, since the IH professional does not have to
44 Occupational Health & Safety | OCTOBER 2018
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