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)
- Physiological/stress — raised blood pressure, increased heart rate, elevated stress hormones; long-term cardiovascular effects.
- Interference with communication — masks speech and warning signals → safety hazard (workers cannot hear alarms or instructions).
- Reduced performance — distraction, impaired concentration, more errors and accidents.
- Annoyance and irritability, fatigue.
- Sleep disturbance (for shift patterns/off-work recovery).
- Tinnitus (ringing) and temporary threshold shift.
(ii) Audiology and Audiogram
- Audiology — the science/clinical practice concerned with hearing: the measurement, assessment, diagnosis and management of hearing function and disorders. In industry it underpins hearing-conservation programs (baseline and periodic testing).
- Audiogram — a graph of a person's hearing threshold (in dB of hearing level) versus frequency (Hz), obtained by audiometry. It shows the quietest sound audible at each frequency; a dip (typically near 4000 Hz—the "noise notch") indicates noise-induced hearing loss. Comparing periodic audiograms to a baseline detects a standard threshold shift early.
(iii) Daily Noise Dose Calculation
The combined dose is the sum of the fractions of each exposure to its permissible duration: $D = \sum \dfrac{C_i}{T_i}$.
$$D = \frac{4}{16} + \frac{2}{8} + \frac{1}{4} + \frac{1}{2} = 0.25 + 0.25 + 0.25 + 0.50 = 1.25$$
$$D = \boxed{1.25 = 125\%}$$
The durations the question supplies are the 5 dB exchange rule anchored at 85 dBA = 8 h ($T = 8/2^{(L-85)/5}$ gives 16, 8, 4 and 2 hr at 80, 85, 90 and 95 dBA). The equivalent 8-hour time-weighted average must therefore be read off the same 85 dBA anchor:
$$TWA = 85 + 16.61\log_{10}(D) = 85 + 16.61\log_{10}(1.25) = 85 + 1.61 \approx 86.6\ \text{dBA}$$
Cross-check on OSHA’s own Table G-16a, whose reference durations ($T = 8/2^{(L-90)/5}$: 32, 16, 8 and 4 hr) are exactly double the ones printed here: the same exposure scores $D = \tfrac{4}{32}+\tfrac{2}{16}+\tfrac{1}{8}+\tfrac{1}{4} = 0.625$ and $TWA = 90 + 16.61\log_{10}(0.625) = 86.6$ dBA — the same level, as it must be. The dose is criterion-dependent; the TWA it encodes is not.
| Level | Exposure $C_i$ | Permissible $T_i$ | $C_i/T_i$ |
|---|
| 80 dBA | 4 hr | 16 hr | 0.25 |
| 85 dBA | 2 hr | 8 hr | 0.25 |
| 90 dBA | 1 hr | 4 hr | 0.25 |
| 95 dBA | 1 hr | 2 hr | 0.50 |
| Total dose D | 1.25 |
Is it acceptable? No. Since $D = 1.25 > 1.0$ (125% > 100%), the daily exposure exceeds the permissible limit—it is NOT acceptable (equivalent TWA ≈ 86.6 dBA > the 85 dBA criterion on which the supplied table is based).
What should be done (in hierarchy order):
- Engineering controls (preferred) — reduce noise at source: enclosures, mufflers/silencers, vibration isolation, damping, maintenance of worn parts, quieter equipment, barriers.
- Administrative controls — limit exposure time, rotate workers, schedule noisy work when fewer are present, provide quiet rest areas.
- Hearing protection (PPE) — earplugs/earmuffs with adequate attenuation (last resort until controls reduce exposure).
- Implement a hearing-conservation program — noise monitoring, audiometric testing, training, and signage.
Canadian context. The durations the question labels “OSHA” keep OSHA’s 5 dB exchange rate but are anchored at 85 dBA = 8 h, not at OSHA’s own 90 dBA — each printed value is half the corresponding Table G-16a duration. Most Canadian jurisdictions set an 85 dBA Lex(8h) limit with a 3 dB exchange rate (per CSA Z107.56 and provincial OH&S regulations; Québec still uses 90 dBA / 5 dB), so the limit agrees with the supplied table but the exchange rate does not. Under a 3 dB rule every +3 dBA halves the allowed time (25.4, 8, 2.52 and 0.79 hr at 80, 85, 90 and 95 dBA), giving $D = 2.06$ and $L_{ex(8h)} = 85 + 10\log_{10}(2.06) = 88.1$ dBA — this same exposure accumulates dose even faster and is still unacceptable, so the Canadian assessment is the more protective one. Above, the dose is computed with the table exactly as the question gives it.
