Noise Exposure Estimator: the TWA and the Dose

Add a row per task or per measured level — the tool calculates the 8-hour TWA, the dose percentage, and tells you the action: below the lower line, the action level, or the limit, with the control and the surveillance that each position requires. The formula is shown with the worked example, so the number is not a black box.

How to use the tool

1. Add a row per exposure — the level in dBA (the reading you took) and the time in hours
2. Enter each exposure for the day — the task, the level, the time; the tool sums the day
3. Read the TWA — the 8-hour time-weighted average, the value the limit is set against
4. Read the dose — the OSHA percentage: 100% is the OSHA limit, above it is the exceedance
5. Take the action for the position — below 80: monitor; 80–85: the opportunity for control; 85: the UK action level (conservation programme); 90: the OSHA PEL (protection, the programme, the surveillance)

Worked example

A day in a plant: 4 hours at 88 dBA (the press line), 2 hours at 94 dBA (the grinding bay), 2 hours at 80 dBA (the assembly). The tool: TWA = 90 + 16.61 × log10( (4/8)×10^(0.0602×(88−90)) + (2/8)×10^(0.0602×(94−90)) + (2/8)×10^(0.0602×(80−90)) ) ≈ 89.1 dBA. Dose: 88% — below the OSHA limit, above the UK action level. Position: above the UK upper value (85) — the control duty, the conservation programme and the audiometry apply; below the OSHA PEL (90) but the dose is climbing — the rotation or the extract on the grinder is the engineering move that brings the TWA under 85, and the audiometric surveillance is in place for the exposed group. The re-measure after the extract is the proof.

Standard & legal context

The UK Control of Noise at Work Regulations 2005 set the lower exposure value at 80 dBA L_Aeq,8h and the upper at 85 dBA L_Aeq,8h — the upper triggers the control duty, the hearing conservation programme and the audiometry; the peak limit is 137 dB C. OSHA 29 CFR 1910.95 sets the 90 dBA TWA action level and PEL with the 5 dBA exchange rate. The 85 in the UK and the 90 in the US are two different energy logics, and both are in the table on this page. The TWA formula is the same arithmetic both use; the tool calculates both, and the stricter governs the workplace.

Common mistakes

1. Averaging the levels instead of the energy — noise adds in energy (the 10^(0.0324L) term), not in arithmetic; the average of 80 and 100 is not 90
2. Using the peak as the exposure — the peak matters (the 137 dB C limit, the impulse noise), but the TWA is the exposure the limit is set on
3. Measuring the plant and not the worker — the exposure is the level at the ear for the time at the task; the spot meter at the fence is not the TWA
4. Stopping at ear plugs issued — the protection is the last layer; the rotation, the extract, the enclosure are the ones that move the TWA

TWA Calculation (Time Weighted Average): Worked Examples

The 8-hour TWA is the single number that the regulation compares against the action level, and the calculation runs on the doubling rule — the UK doubles the dose every 3 dB, the US every 5 dB, which is why the same shift can sit on different sides of the limit on the two sides of the Atlantic. The worked example: a shift with 85 dB(A) for 4 hours and 88 dB(A) for 4 hours — the 85 contributes half the 8-hour dose, the 88 contributes about 0.38, the total is about 0.63 of the 8-hour equivalent, and the TWA works out to about 86.7 dB(A) — over the UK upper action level of 85, and the hearing protection zone is the duty. The estimator on this page runs the same formula with the substitution shown, so the number is defensible on the audit and the method is visible on the form.