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
• Mar 01, 2020

Workplace noise measurements are a critical aspect for today’s industrial hygienist and health and safety professionals. High levels of noise still continue 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 establish 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 traditionally 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 microphone 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 eliminates 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 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 microphone capsule for a new one to maintain the calibration ability of the whole instrument.

Some new dosimeters now feature solid state microphones using the MEMS technique (Micro Electro 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 microphone 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 potential 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 occurrence.

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 software 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 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 happening 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. Threshold triggered recordings help to limit the actual recording 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 frequency 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 for sure when and where they occurred. It also means that the true noise sources are measured at the same time as the dose percentage, and the average noise levels are being collected. This leads to much more useful and accurate determinations of the likely harmful exposure frequencies and hence a more accurate choice of the right hearing protectors for the worker.

Vibration Sensors and Other Improvements
There is always the possibility that the microphone may get knocked during the measurements, leading to erroneous results for the true peak level and possibly over-estimating the time average result. Some modern dosimeters feature a self-monitoring vibration sensor that marks samples during the run when the dosimeter is knocked or bumped accidentally (or deliberately) and these results can be identified in the software and removed after downloading.

The vibration sensor in the dosimeter serves another useful purpose: monitoring when the instrument is not being worn. This helps to alert when the operator may have taken it off and set it down. The dosimeters can detect when the instrument is being worn even if the operator is just sitting at a bench working normally. These dosimeters can provide very useful information to the IH looking to be sure that nothing out of the ordinary has occurred.

In the beginning of personal noise dosimetry measurement, the available instruments simply provided an overall noise dose percentage result based on one of the protocols specified by the regulatory bodies around the world. Gradually, more results were included to better represent the measurement to give more insight into what had occurred including peak levels; maximum levels; time average levels; and normalized, time-weighted average levels. Different protocol measurements required repeats of the measurement to gain the right information about a worker’s exposure patterns, and this could take two or three days to collect.

Current noise dosimeters feature multiple “virtual noise dosimeters” inside a single instrument by carefully splitting the acoustic path into two or three or more separate profiles configurable by the user. This way, a single measurement can now produce results calculated by the different required methods: OSHA, ACGIH, ISO, or even a user’s own in-house protocol. One measurement saves time and effort by collecting all results in parallel.

Newer dosimeters also feature Bluetooth communications for use with a dedicated app on a smartphone or tablet to review the accumulated noise exposure any time during the shift without having to interfere with the worker. This is a great practical advantage and enables the IH to see what the exposure levels are in the moment without worker interference. If an above-the-limit noise is identified, workers can be given alternative, quieter tasks for the remainder of the day.

Conclusion

This article originally appeared in the March 2020 issue of Occupational Health & Safety.