Stone Workshop: Is the Earplug Enough?
A stone-cutting workshop employs 12 workers. The main saw runs 6 hours per shift at 98 dBA at the operator's ear; a hand grinder runs 2 hours at 91 dBA. The remaining time is spent in the same room without other significant noise. The owner supplies earplugs labelled NRR 25 and says this makes the workplace safe. A worker complains of ringing in the ears after work. Measurements were made with a calibrated sound-level meter on the A-weighted, slow response. The workshop operates 8 hours per day, 6 days per week. The occupational limit used locally is 85 dBA as an 8-hour equivalent. The worker wants to know whether his hearing is at risk and whether the earplugs are enough.
1. Compute the 8-hour equivalent sound level from the two exposures and decide whether the occupational limit is exceeded.
Read the worked answer
Use the equal-energy formula: Leq,8h = 10 log10[(t1/8) × 10^(L1/10) + (t2/8) × 10^(L2/10)].
For the saw: (6/8) × 10^(98/10) = 0.75 × 6.31 × 10^9 = 4.73 × 10^9. For the grinder: (2/8) × 10^(91/10) = 0.25 × 1.26 × 10^9 = 0.315 × 10^9. Sum = 5.05 × 10^9. 10 log10(5.05 × 10^9) = 10 × 9.703 = 97.0 dBA.
The 8-hour equivalent is about 97 dBA, which exceeds the 85 dBA limit by 12 dB. In energy terms, 12 dB is a factor of about 16 above the limit. The saw dominates the exposure; reducing saw noise or saw time will have the largest effect. The grinder alone would give about 91 dBA for 2 hours, which is already an 8-hour equivalent of about 85 dBA, so even the “quieter” task is at the limit.
2. Estimate the protected exposure using the NRR 25 rating with the common derating (NRR − 7)/2, and explain why the labelled NRR may overestimate protection.
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Derated attenuation = (25 − 7)/2 = 9 dB. Protected level ≈ 97 − 9 = 88 dBA, which is still above 85 dBA. The earplug therefore does not make the workplace safe as claimed. The calculation also assumes perfect fit and continuous use; if the plug is removed for even part of the shift, protection falls sharply.
The labelled NRR is measured under laboratory conditions and is not a guarantee at the individual's ear. Real-world attenuation is reduced by poor insertion, loose fit, jaw movement, spectacle frames, beards, and the need to communicate. The NRR is also a single-number rating and may not match the frequency spectrum of stone-cutting noise. In addition, the worker's tinnitus suggests current exposure is already causing auditory stress. Earplugs are the last line of defence, not the first. The correct conclusion is that hearing protection alone is insufficient; engineering and administrative controls are needed.
3. Recommend controls in order of priority and advise the worker about the ringing in his ears.
Read the worked answer
First, eliminate or reduce noise at source: buy quieter saws or fit acoustic enclosures, mount saws on anti-vibration bases, and maintain blades and bearings. Second, separate the noisy processes: put the saw in an enclosed room with sound-absorbing surfaces and keep workers out except when operating it; use remote operation where possible. Third, reduce exposure time by rotating tasks and scheduling grinding away from the saw. Fourth, provide properly fitted hearing protection with a tested attenuation, train workers in insertion, and check fit. Finally, introduce audiometric testing at baseline and annually, with a record of shifts and exposure.
The ringing is a warning symptom and may indicate temporary threshold shift or early noise-induced hearing loss. The worker should report it, be tested after a quiet period, and not accept “earplugs are enough” as an answer. He should use hearing protection consistently in the workshop and outside if exposed to loud tools, and seek medical review if tinnitus persists, is unilateral, or is accompanied by hearing loss or dizziness. The employer should act on the exposure calculation, not only on the label.