Noise Dose in a Small Corrugated-Metal Workshop: Reverberant Build-Up, Daily Dose from Hammering, and What Cheap Treatments Actually Buy

Corrugated metal makes a small workshop loud in a way that a single sound level at the tool does not capture, because hard parallel surfaces return the sound to the room instead of absorbing it. This paper computes the consequence for a 5.0 by 4.0 by 2.7 m steel workshop (54.0 m3) in which hammering has a stated free-field level of 95 dBA at 1 m. The room has only 7.83 sabins of absorption and a Sabine reverberation time of 1.11 seconds; the reverberant level reaches 103.08 dBA and the combined level at the operator's ear, 0.5 m from the work, is 105.18 dBA. Under the NIOSH criterion of 85 dBA for 8 hours with a 3 dB exchange rate, the permitted time at that level is 0.076 hours, the full daily dose is reached after 3.6 minutes of hammering when the rest of the day is spent at 78 dBA, 15 minutes of hammering gives a dose of 350.2%, and 60 minutes gives 1341.2%. Cheap room treatments disappoint for the operator: 30 m2 of mineral wool, costing $540 in materials, lowers the operator level by only 2.71 dB, because the direct sound at 0.5 m dominates once the reverberant field is reduced; moving a bystander from 1 m to 3 m buys only 0.55 dB for the same reason in reverse, the reverberant field being nearly uniform. A $40 absorptive screen ranks first among room treatments at 2.02 dB per $100 spent, plywood lining ranks last at 0.19 dB per $100, and hearing protection at $25, giving an effective 20 dB, is in a different class altogether — yet even protected, a full 8 hours of hammering gives a dose of 104.2%. The unprotected dose bound is therefore not a matter of treating the room: a few minutes of hammering in this room exhausts the daily criterion.