The formulas
The calculator uses the dose method OSHA describes in 29 CFR 1910.95 and its Appendix A. L is a sound level in dBA, C is the hours spent at that level, and T is the hours allowed at that level.
- T = 8 ÷ 2^((L − 90) ÷ 5). Eight hours at 90 dBA, and the time halves with every 5 dBA more, so 4 hours at 95 and 2 hours at 100. Below 90 the time doubles: 16 hours at 85.
- Dose D = 100 × (C1 ÷ T1 + C2 ÷ T2 + …). Each term is the share of the allowed time you used at that level. A dose of 100 means all of it.
- TWA = 16.61 × log10(D ÷ 100) + 90. The constant 16.61 comes from the 5 dB exchange rate. A dose of 100 gives 90 dBA, and a dose of 50 gives 85 dBA.
This tool gives the dose for the 85 dBA action-level basis: sound from 80 dBA counts, and rows under 80 dBA get zero weight because they don't count toward the dose (see 1910.95 and its Appendix A). A different counting threshold applies to the 90 dBA limit, so read the rule before using a result for that comparison. Enter sound levels as measured, with no credit for hearing protection.
Worked example 1: two levels in one shift
4 hours at 92 dBA and 2 hours at 95 dBA. T at 92 = 8 ÷ 2^0.4 = 6.063 hours, so C ÷ T = 4 ÷ 6.063 = 0.660. T at 95 = 4 hours, so C ÷ T = 2 ÷ 4 = 0.500. The sum is 1.160, so the dose is 116.0%. TWA = 16.61 × log10(1.160) + 90 = 91.1 dBA.
Worked example 2: three counted levels and one quiet period
4 hours at 85 dBA, 3 hours at 91 dBA, 0.5 hours at 100 dBA and 2 hours at 72 dBA. T at 85 = 16 hours, so C ÷ T = 0.250. T at 91 = 8 ÷ 2^0.2 = 6.964 hours, so C ÷ T = 0.431. T at 100 = 2 hours, so C ÷ T = 0.250. The 72 dBA row is under 80 dBA and gets zero weight (counting it would wrongly add 2 ÷ 97.0 = 0.021). The sum is 0.931, so the dose is 93.1% and the TWA is 16.61 × log10(0.931) + 90 = 89.5 dBA. The hours and levels are invented. These are original arithmetic examples, not exam questions.
How this shows up in CHST study
The CHST blueprint lists noise among the health hazards in its first domain, and it expects you to know how exposure is measured and compared with a limit. Be able to say what the 5 dB exchange rate does to allowed time, to add C ÷ T terms for a mixed day, and to tell a dose of 50 from a dose of 100. Construction work is covered by 1926.52, which uses the same 90 dBA, 8-hour, 5 dB table; the 85 dBA action level comes from 1910.95. This tool follows the 1910.95 arithmetic. See the silica, lead and noise guide for the construction rule and the CHST formulas guide for the other calculations.
Noise dose questions
Why does the time halve every 5 dBA?
That is the 5 dB exchange rate in OSHA's noise rule: 8 hours at 90 dBA, 4 hours at 95, 2 hours at 100. Some other agencies and standards use 3 dB, so check which rate a question or a document assumes.
What dose equals a TWA of 90 dBA?
A dose of 100 percent. It is 8 hours at 90 dBA, or any mix of counted levels whose C divided by T terms add to 1.0. Rows under 80 dBA add nothing. A dose of 50 percent works out to a TWA of 85 dBA.
Does hearing protection change the dose?
Not in this calculator. Enter the sound levels measured at the worker's ear position with no allowance for earplugs or earmuffs.