One of the things I love about my job is that I'm always learning about our rental instruments and how they're used in testing and reporting.
Last week, a rental customer called me about some individual worker reports he was reviewing from the personal noise dosimeters he'd rented from us. One of the results, an item labeled LCpk, jumped out at him because almost across the board, the result was in the 140 dBA range. What could possibly be so loud as to qualify for what OSHA identifies as the "threshold of pain"?
I didn't have the answer off the top of my head, so I did a little research, and now it all makes sense.
So first, let's start with a few definitions:
A-Weighting: Follows the frequency sensitivity of the human ear at low levels. This is the most commonly used weighting scale, as it also predicts quite well the damage risk of the ear. Sound level meters set to the A-weighting scale will filter out much of the low-frequency noise they measure, similar to the response of the human ear. Noise measurements made with the A-weighting scale are designated dBA.
B-Weighting: Follows the frequency sensitivity of the human ear at moderate levels, used in the past for predicting performance of loudspeakers and stereos, but not industrial noise.
C-Weighting: Follows the frequency sensitivity of the human ear at very high noise levels. The C-weighting scale is quite flat, and therefore includes much more of the low-frequency range of sounds than the A and B scales. The C-weighted scale is used ... particularly for characterizing low-frequency sounds capable of inducing vibrations in buildings or other structures. C-weighted measurements are expressed as dBC or dB(C).
Peak sound pressure level (Lpk) is a measure of the maximum instantaneous sound pressure at a specified location.
Okay, so do I use A-weighting or C-weighting?
According to OSHA's Noise standard (29 CFR 1910.95), the noise dosimeter for making compliance measurements must be set up to record noise exposure using the following criteria:
- Exchange rate: 5 dB
- Frequency weighting: A
- Response: slow
- Criterion level: 85 dBA (Hearing Conservation) or 90 dBA (Administrative and Engineering Controls).
- Threshold: 80 dBA (Hearing Conservation) or 90 dBA (Administrative and Engineering Controls).
So if I'm using A-weighted frequency for my workshift noise sampling, what does LCpk tell me? And should I be concerned?
Mostly, it tells you that some loud instantaneous noises are present. But with a reading around 140 dBA, how are workers not complaining? Perhaps because they're not even noticing the noise. And OSHA explains why:
The peak circuitry is very sensitive. Test this by simply blowing across the microphone. You will notice that the peak reading may be 120 dB or greater. When you take a long‐term noise sample (such as a typical 8‐hour workday sample for OSHA compliance), the peak level is often very high. Because brushing the microphone over a shirt collar or accidentally bumping it can cause such a high reading, the user must be careful not to place too much emphasis on the reading. From OSHA Technical Manual (OTM), Appendix A, Page A-4
Also in the OSHA Technical Manual, Section III, Chapter 5 (Equipment), it suggests the following:
Avoid positioning the microphone where it could become enfolded in clothing or rub against cloth or other materials, both of which could influence the results.
So the short answer is this:
That average C-weighted peak is the measure of a short-term loud noise picked up by the dosimeter's microphone, but it may not be a measurement you need to be concerned about, as the simple act of clothing rubbing against the mic can produce sound levels in the high decibel range.
Learn more about personal noise dosimeters and area sound level meters at RaecoRents.com, and read more in our Sound and Noise topics category.