Calculate noise TWA from sound level and exposure time, or solve for the missing sound level or duration using OSHA 8-hour exposure rules.
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TWA Formula
This TWA calculator uses the OSHA noise dose method for one steady sound level over one exposure time. It uses a 90 dBA criterion level, an 8-hour reference duration, and a 5 dB exchange rate.
T(L) = 8 / 2^((L - 90)/5)
D = 100 * (C / T(L))
TWA = 16.61 * log10(D / 100) + 90
To solve for exposure time when sound level and TWA are known:
D = 100 * 10^((TWA - 90)/16.61)
C = (D / 100) * T(L)
To solve for sound level when exposure time and TWA are known:
D = 100 * 10^((TWA - 90)/16.61)
L = 90 + 5 * log2((D / 100) * (8 / C))
- TWA = time-weighted average noise level, usually in dBA
- L = sound level entered in the calculator
- C = exposure time at that sound level, in hours
- T(L) = OSHA allowable exposure time at sound level L, in hours
- D = noise dose as a percent
- log10 = base-10 logarithm
- log2 = base-2 logarithm
If you leave TWA blank, the calculator finds the OSHA dose from the sound level and exposure time, then converts that dose to TWA. If you leave exposure time blank, it converts the TWA to dose and finds the time that would produce that dose. If you leave sound level blank, it finds the steady sound level that matches the entered TWA and exposure time.
OSHA Noise Exposure Reference Values
The table below shows common OSHA allowable exposure times using the 5 dB exchange rate used by this calculator.
| Sound level | Allowable time | Dose if exposed for that time |
|---|---|---|
| 85 dBA | 16 hours | 100% |
| 90 dBA | 8 hours | 100% |
| 95 dBA | 4 hours | 100% |
| 100 dBA | 2 hours | 100% |
| 105 dBA | 1 hour | 100% |
| 110 dBA | 30 minutes | 100% |
| 115 dBA | 15 minutes | 100% |
These values are based on the same exchange-rate relationship used in the formula. They are not a full noise survey by themselves.
TWA Result Guide
| TWA result | Common OSHA meaning |
|---|---|
| Below 85 dBA | Below the OSHA hearing conservation action level for an 8-hour TWA. |
| 85 dBA to below 90 dBA | At or above the OSHA action level. Hearing conservation requirements may apply. |
| 90 dBA or higher | At or above the OSHA permissible exposure limit for an 8-hour TWA. |
Example Problems
Example 1: Find TWA from sound level and time.
You are exposed to 95 dBA for 4 hours.
T(95) = 8 / 2^((95 - 90)/5) = 4 hr
D = 100 * (4 / 4) = 100%
TWA = 16.61 * log10(100 / 100) + 90 = 90 dBA
The TWA is 90 dBA.
Example 2: Find sound level from TWA and time.
You know the TWA is 85 dBA for an 8-hour exposure.
D = 100 * 10^((85 - 90)/16.61) = 50%
L = 90 + 5 * log2((50 / 100) * (8 / 8)) = 85 dBA
The steady sound level is 85 dBA.
FAQ
Is TWA the same as the sound level reading?
No. A sound level reading is the noise level at a moment or over a measurement period. TWA adjusts the exposure to an 8-hour workday basis. A high sound level for a short time can have the same TWA as a lower sound level for a longer time.
Should you use dB, dBA, or dBC?
Occupational noise TWA is usually based on A-weighted sound levels, written as dBA. The calculator treats dB, dBA, and dBC as numeric sound-level labels and does not convert between frequency weightings. Use the same weighting that matches your measurement and standard. For OSHA noise TWA, dBA is typically used.
Can this calculate multiple noise exposures in one shift?
This calculator is set up for one sound level and one exposure time. If a shift has several different noise levels, calculate the dose contribution for each period and add the doses before converting total dose to TWA.
