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11-CS-2 Engineering Communications · December 2014

Question 6 of 7: Industrial Noise — Effects, Audiology, and Noise Dose

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

National Exams — December 2014 — 11-CS-2 Engineering in Society: Health and Safety. Closed book; approved calculator permitted. Any five of the seven questions constitute a complete paper; all questions are of equal value. Answers are written in point form but fully, as instructed. Complete answers to all seven questions follow.

Question 6: Industrial Noise — Effects, Audiology, and Noise Dose (20 marks: 6/6/8)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(i) Detrimental Effects of Noise (other than hearing loss)

(ii) Audiology and Audiogram

(iii) Daily Noise Dose

$$D = \sum \frac{C_i}{T_i} = \frac{4}{16} + \frac{2}{8} + \frac{1}{4} + \frac{1}{2} = 0.25 + 0.25 + 0.25 + 0.50 = \boxed{1.25 = 125\%}$$
Level$C_i$$T_i$$C_i/T_i$
80 dBA4 hr16 hr0.25
85 dBA2 hr8 hr0.25
90 dBA1 hr4 hr0.25
95 dBA1 hr2 hr0.50
Total D1.25

Equivalent 8-hour TWA. The anchor of the TWA formula must match the table the question supplies. These durations are the 5 dB exchange rule anchored at 85 dBA = 8 h (each +5 dBA halves the time: 16, 8, 4, 2 h), so the criterion level is 85 dBA, not 90:

$$\text{TWA} = 85 + 16.61\log_{10}(D) = 85 + 16.61\log_{10}(1.25) = 85 + 1.61 = \boxed{86.6\ \text{dBA}}$$

Cross-check on OSHA's own Table G-16a, whose durations are double these at every level (80 dBA–32 h, 85–16, 90–8, 95–4): the same exposure scores $D = 0.125 + 0.125 + 0.125 + 0.25 = 0.625$, and $90 + 16.61\log_{10}(0.625) = 86.6$ dBA — the identical TWA. The dose depends on which criterion the table is anchored to; the TWA it represents does not. That is the check that settles the anchor: using $90 + 16.61\log_{10}(1.25) = 91.6$ dBA would mix this paper's 85 dBA-anchored dose with OSHA's 90 dBA-anchored formula and overstate the exposure by the full 5 dBA.

Acceptable? No—$D = 1.25 > 1.0$ (125% > 100%); the exposure exceeds the permissible limit of the supplied table (TWA ≈ 86.6 dBA > the 85 dBA criterion that table is built on).

What to do (hierarchy): engineering controls first (enclosures, mufflers/silencers, vibration isolation, maintenance, quieter equipment); then administrative controls (reduce exposure time, rotation); then hearing protection (PPE); all within a hearing-conservation program (monitoring, audiometry, training, signage).

Canadian context. The permissible-duration table the question supplies (labelled “OSHA”) is a 5 dB exchange rate anchored at an 85 dBA criterion — exactly half OSHA Table G-16a at every level. Most Canadian jurisdictions set the same 85 dBA Lex(8h) limit but with a 3 dB exchange rate (per CSA Z107.56 and provincial OH&S regulations; Québec still uses 90 dBA / 5 dB), so what differs is the exchange rate, not the limit. Under a 3 dB rule every +3 dBA halves the allowed time, so this same exposure accumulates dose faster still — $D = 2.06$, i.e. $L_{ex,8h} = 85 + 10\log_{10}(2.06) = 88.1$ dBA — and is unacceptable by a wider margin; the Canadian assessment is the more protective. Above, the dose is computed with the table exactly as the question gives it.