OpenSpecEngineering

Adds up a worker’s noise exposure over a shift. From up to four segments, each a sound level in dBA and a duration, it gives the permissible time at each level, the daily noise dose and the 8-hour time-weighted average (TWA). You can rate the day under the OSHA rule (90 dBA criterion, 5 dB exchange rate) [1] or the NIOSH recommendation (85 dBA criterion, 3 dB exchange rate) [2]. Safety engineers and industrial hygienists use it to judge noise-meter readings.

The governing equations

T_i = 8 / 2^{(L_i − L_c)/ER}    D = Σ C_i/T_i

TWA = L_c + (ER / log₁₀2)·log₁₀D

L_i is the A-weighted sound level of segment i, in dBA, and C_i the hours spent at it. T_i is the permissible time at that level. L_c is the criterion level, 90 dBA for OSHA and 85 dBA for NIOSH, which is allowed for 8 hours. ER is the exchange rate: each ER decibels above L_c halves the permissible time. D is the dose as a fraction, shown as a percent. The TWA is the steady level that would give the same dose in 8 hours. ER/log₁₀2 is 16.61 for OSHA [1]. For NIOSH it is 9.97, where [2] rounds it to 10, so the page’s NIOSH TWA can be a few hundredths of a dB lower.

Using it and common mistakes

• OSHA uses two thresholds [1]. For the hearing conservation action level (a TWA of 85 dBA, a 50 % dose), it counts levels from 80 dBA up. For the permissible exposure limit (a 100 % dose), it counts only levels of 90 dBA and up. The page counts every segment you enter, so leave out the segments below the threshold you are checking.
• Enter A-weighted levels on slow response, as a sound level meter or dosimeter reports them. Peaks and impulse noise need a separate check: OSHA allows no exposure above 140 dB peak.
• A shift longer than 8 hours is fine, up to 24 hours. The TWA still refers to 8 hours.

Worked example

A maintenance mechanic spends 2 hours at a press line measured at 95 dBA, 3 hours in the compressor room at 90 dBA and 3 hours in the shop at 82 dBA. Check the day against the OSHA rules.

1. Choose Rule OSHA (5 dB exchange, 90 dBA criterion).
2. Enter Segment 1 level 95 and duration 2, and Segment 2 level 90 and duration 3.
3. Enter Segment 3 level 82 and duration 3, and leave Segment 4 at 0.
4. Press Solve. The Solution shows: 95 dBA — permissible time: 4 hr
   90 dBA — permissible time: 8 hr
   82 dBA — permissible time: 24.2515 hr
   Total exposure duration: 8 hr
   Noise dose: 99.8704 %
   8-hour time-weighted average: 89.9906 dBA
   Status: At or below the limit
   OSHA note: TWA ≥ 85 dBA — hearing conservation program required

Check: 2/4 + 3/8 + 3/24.25 = 0.500 + 0.375 + 0.124 = 0.999. With the 82 dBA hours counted, this is the action-level dose, and the mechanic needs the hearing conservation program. For the permissible exposure limit, set Segment 3 to 0: the page then gives a Noise dose of 87.5 %, within the limit. Under the NIOSH rule the full day gives a 389.789 % dose.