Hearing loss from noise is cumulative, permanent and painless while it happens. So the regulation is not about how loud, but about how loud for how long: $T = \dfrac{8}{2^{(L - L_c)/q}}$ — T equals eight over two to the L minus L-c, over q.

The letters. $T$ is the allowable exposure time in hours, the point at which a full daily dose has been spent. $L$ is the steady A-weighted level at the ear in dBA. $L_c$ is the criterion level — the level a worker may be exposed to for a full eight hours. And $q$ is the exchange rate: the number of decibels that halves the permitted time.

Those last two are not physics. They are policy, and they differ. NIOSH and most of Canada use 85 dBA with a 3 dB exchange — three decibels is a doubling of energy, so the rule follows the physics. OSHA uses 90 dBA with a 5 dB exchange, a compromise written in 1971 and still in force. Run the same worker beside the same machine at 100 dBA through both: NIOSH allows $8/2^{5} = 0.25$ h — fifteen minutes. OSHA allows $8/2^{2} = 2$ h. Eight times the exposure, same ear. Which rule governs your site is not a detail, and it is the first question to ask on an unfamiliar job.

Read backwards it sets a ceiling instead of a clock: $L = L_c + q\log_{2}\!\left(\dfrac{8}{T}\right)$. The logarithm to base two is just how many times have the hours halved from eight, and it is almost always countable on your fingers: one hour is three halvings, so on the 3 dB rule the ceiling is $85 + 9 = 94$ dBA.

The trap is treating the exchange as linear. Halvings compound: four steps above the criterion is a sixteenth of the time, not a fifth of it. And a nugget for the field — because 6 dB is one doubling of distance and 3 dB is one halving of time, stepping twice as far from a point source buys four times the allowed hours. Distance is the cheapest hearing protection ever invented, and it needs no fitting, no training and no inspection.