Measuring noise exposure in HD
August 19, 2020
By
OHS Canada
Advances continue in noise dosimetry
Recent advances in digital signal processing technology have led to the introduction of new dosimetry methods. (Photo: Courtesy of HGC Engineering)
By Rob Stevens
Noise dosimetry is one of the most important methods in assessing workplace noise exposure.
Dosimetry involves attaching a small sound level monitor with data logging capability on a worker during a full work shift. Ideally, the sound data collected represents the noise exposure of that worker.
Unfortunately, the logged results frequently overstate workers’ noise exposures. Dosimetry is prone to capturing irrelevant “pseudo-noises” — such as bumps against the microphone, rustling of clothing or the worker’s own voice, which the dosimeter alone cannot differentiate from legitimate health-impacting workplace noise.
Including these pseudo-noises as part of the final results can inflate the data numbers and suggest the workplace is louder than it actually is, leading to unwarranted costs in pursuing noise control efforts for what is actually a non-existent workplace health concern.
Recent advances in digital signal processing technology have led to the introduction of new dosimetry methods, which increase accuracy and actually enable identification of key sources of excessive noise in the workplace. In some jurisdictions, the governing standards now recommend using these new methods to better assess noise exposure.
Workplace noise management
Broadly speaking, managing noise effectively in the workplace involves:
• determining the noise exposure levels of workers and the sound levels throughout the workplace
• understanding where the noise exceeds the limits and then implementing engineered noise controls, if practicable
• recommending the most appropriate hearing protection devices and enforcing their use for areas where the sound levels cannot feasibly be reduced.
A shorthand way to think of these three components is: workplace noise assessment, workplace noise control and hearing conservation.
Traditionally, dosimetry has only been part of the first step in noise management — workplace noise assessment. If the dosimetry indicates excessive noise exposures, considerable additional work is usually needed toward workplace noise control, including: identifying the specific equipment, operations and activities contributing to elevated noise levels; and investigating developing solutions for engineered noise control measures.
The key benefit of dosimetry is that it can be pre-programmed to run automatically without any intervention, while a worker goes about their day wearing the device.
In this way, a dosimeter can gather many hours of continuous sound data without the ongoing presence or participation of a technician or acoustical consultant. So, the risk of missing occasional or intermittent noisy events or other variations in sound level over time is reduced.
The automated nature of dosimetry is also its weakness.
The dosimeter has no real built-in “smarts” and so, unlike an acoustical expert, it cannot use its own field experience and judgment to recognize anomalous noises and exclude them to avoid “false,” inflated readings.
Moreover, traditional dosimeters utilize primarily analog electronics, which cannot process or store detailed acoustical information. So, they offer little information about the characteristics of the noise, which could be useful to identify the dominant sources of the noise.
Towards high-definition dosimetry
Several years ago, HGC Engineering began to research better ways to do dosimetry measurements, ultimately adopting an approach dubbed high-definition (HD) dosimetry.
For a decade or more, there have been digital sound level meters, much larger than a dosimeter, which can digitally record the actual audio in a workplace noise survey at the same time that it captures the sound level data. And they can process acoustic frequency information, measuring in full-octave or 1/3-octave frequency bands.
If we chose to use one in the field, we would either have to strap a cumbersome digital sound level meter to a worker with the microphone on a cable, pinned to the lapel or shoulder, or we would use a dosimeter alongside a digital audio recorder.
By recording audio and configuring the instrument to store the results in fast time steps — typically once per second — we were able to view the graph of sound level versus time (often called a “time-history”) in post-processing, find any peaks in sound level, and then listen to the audio recording to identify the type of sound.
This new approach confirmed exactly what we had long suspected — traditional dosimetry frequently overestimates true noise exposure levels. But we were surprised by the extent of these overestimates.
By clipping any of the noise irregularities out of the data, we found that in a considerable number of cases, the unfiltered sound data showed noise exposure levels exceeding the governing limits, while the corrected results were well within the limits.
Given this degree of divergent
results, the bottom-line consequences to a business not using HD dosimetry can be significant.
Digital dosimetry
Fortunately within the last few years, some instrumentation manufacturers have introduced fully digital dosimeters, which can gather calibrated audio recordings, measure in full-octave or 1/3-octave frequency bands, and log the results with very fine time resolution.
The accompanying post-processing software easily allows the user to view the time-history graphs and listen to the synched audio recording at the same time, while the cursor scrolls through the graph. The user can then also highlight and clip out atypical events, or group together similar acoustical events and calculate cumulative exposure levels from different activities or noise sources.
Workplace noise standards are evolving to encourage the use of these improved method.
In Canada, for example, the newly revised CSA Standard Z107.56-18 “Measurement of noise exposure” recognizes the limitations of traditional dosimetry and provides corrective recommendations:
4.2.1 – Concurrent measurement with octave or 1/3-octave bands should be used to assist with hearing protection selection and noise source identification and control.
4.2.3 – Audio recording capability may be used to assist with the identification and removal of spurious events through post analysis if required.
6.3.1 – Users should be aware that dosimetry measurements can be elevated by the worker’s own voice, if communication with raised vocal effort is a common occurrence on the job.
Additional benefits of HD dosimetry
There are broader benefits than simply excluding extraneous “false,” inflated noises from the data set.
In complex workplace environments that have many diverse noise sources — perhaps components of multi-stage manufacturing processes with interlocked operations — it can be a labour-intensive task to isolate the sound from each item of equipment, or even their sub-components, in order to know which ones contribute most to the noise excesses.
Traditional dosimetry offers little to no useful information about what equipment, activities or processes in the workplace are the prime contributors to noise excesses.
Now the real power of HD dosimetry emerges.
In many cases, the audio recording — together with the synchronized time-history graph — can be used to identify and collate sounds of different activities and then calculate the time-weighted sound exposure levels of the various individual activities occurring throughout the worker’s shift.
If the worker is in an area with cycling process stages, such as granulating, mixing, drying and dispensing — by listening to the recording, analysts can use the software to highlight and sum together all occurrences of each process stage and thereby determine which has the greatest impact on the overall noise exposures.
Or if a worker is performing different tasks throughout the shift (for example: milling, welding, grinding, hammering), analysts can flag each occurrence of those activities and have the software calculate time-weighted sound exposures for each type of activity.
With this functionality, it is easier to prioritize the various activities for noise control.
HD dosimetry yields greater accuracy in assessing workplace noise exposure and provides a wealth of information for later noise control studies, reducing costs and saving time.
In that respect, it is quickly establishing itself as an indispensable next-generation tool for workplace health and safety.
Rob Stevens is a principal acoustical consultant at HGC Engineering in Mississauga, Ont.
