OSHA Safe noise levels

If I had to guess I would say the difference is politics, where some industry needs higher guidelines and will pay to get it (political contributions).

That would kinda be my bet, too. NIOSH seems overly conservative, OSHA overly generous. Maybe we should split the difference but those logarithmic calculations get me down!

Edit_Update: Actually, the basic math is not that complicated. For each halving of allowed exposure time, the NIOSH standard only allows the REL (Recommended Exposure Level) to go up by 3 dB. OSHA more generous. For each halving of the allowed exposure time, OSHA allows the PEL (permitted exposure level) to go up 5 dB, at least up to the 115 dBA level.

@jim_lewis said:

The following graph, G-9 from the OSHA PEL Sound Standards, is interesting.

@jim_lewis said:

Perhaps two things are going on. High frequency sound is more energetic than low frequency sound because you are packing more amplitude peaks into the same time frame

Find out on further reading that I’m obviously in error on the first point. dB SPL is dB SPL no matter what the sound frequency and a measure of the sound pressure outside your ear. It turns out that all the contours are related to the acoustics of the human ear and the bio-mechanics of sound transfer to the inner ear - and high frequency sound is better conducted to the inner ear and more damaging once there than lower frequency sound or much higher frequency sound (the rising part of the curves on the right that I’m referencing in my post above with the contour “equal loudness” curves).

The dBA variable that’s depicted changing in the curve is not a direct measure of damage potential. It’s just measure of dB after the sound is passed through a particular type of filter (A filter ) (once) thought to mimic human hearing sensitivity closely. The A-filter was designed based on the sensitivity of human hearing over a frequency range to pure tones compared to the loudness perceived relative to a 1 KHz standard tone. Sound measured through a A-filter, since it mimics the human range of hearing sensitivity, has been found in extensive hearing damage studies to reasonably reflect the potential for sound damage as a function of frequency. But the problem is is that noise is usually not a pure tone but more like white noise so the A-filter-based standard, an American standard, is considered inadequate by some and in Europe, a different standard based on more general types of noise is used as the basis of filtering to determine damaging noise levels. Another variable is the tests were done with headphones on, I believe, or at least the sound coming from the side, and the more usual assault of noise is from the front so modern day standards have been modified to take that into account. The dBA standard is popular in part because of its historical acceptance and history and dBA-based sound meters are cheap and plentiful.

A-weighting3

A-weighting is a form of frequency weighting and the most commonly used of a family of curves defined in the International standard IEC 61672:2003 and various national standards relating to the measurement of sound pressure level. A-weighting is applied to instrument-measured sound levels in an effort to account for the relative loudness perceived by the human ear, as the ear is less sensitive to low audio frequencies. It is employed by arithmetically adding a table of values, listed by octav

Equal-loudness contour3

An equal-loudness contour is a measure of sound pressure level, over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones. The unit of measurement for loudness levels is the phon and is arrived at by reference to equal-loudness contours. By definition, two sine waves of differing frequencies are said to have equal-loudness level measured in phons if they are perceived as equally loud by the average young person without significant hea

Sorry for errors in my explanation. I think that if I were a Ph.D. in audiology, I would understand a bit better than I do currently!

@Don said:

If I had to guess I would say the difference is politics, where some industry needs higher guidelines

I found a great in-depth article by an audi at etymotic’s on the difference between the OSHA and NIOSH standards. She says straight up that the OSHA standard is based on politics and the NIOSH standard is actually based on real scientific studies. She also points out that Europe, China, and most of the rest of the world follow the NIOSH standard with an 85 dBA limit to exposure for 8 hours and a 3 dBA exchange rate, not 5 dBA exchange rate as for OSHA, for 2-fold reduction in allowed exposure time every time the exposure level rises by the exchange amount. (etymotic’s interest in this is that they want to sell you noise dosimeters)

https://www.etymotic.com/downloads/dl/file/id/47/product/307/noise_exposure_explanation_of_osha_and_niosh_safe_exposure_limits_and_the_importance_of_noise_dosimetry.pdf10

She points out that even the NIOSH standard does not guarantee complete hearing protection. If followed, the standard creators still expect there will be additional hearing loss from the amount of noise exposure allowed and 8% of the exposed population would suffer a hearing sensitivity decrement of > 25 dB (or whatever the cutoff is for defining loss) over a 40-year occupational exposure (or whatever the lifetime work period is - too lazy to look up the “whatever’s”). I think it is her article, too, that points out that these are just limits for the 8-hour work day and in no way take into account what happens during the other 16 hours of the day. And also that the EPA’s recommended limit (Yes! the EPA joins the crowd, too!) for a TWENTY FOUR HOUR day is an average exposure of 70 dBA. So, yep!, if you’ve been working a loud shift at 85 dBA for 8 hours, when you come home, you’d better live in mouse-like quiet and not talk too much to not exceed a 24 hour average of 70 dBA!