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Noise-induced hearing loss: mechanism, the 4000 Hz notch, exposure limits, protection and monitoring
Noise injures basal-turn outer hair cells before anything else, so the early sign is a notch around 3000–6000 Hz with recovery at 8000 Hz. Damage depends on the dose — level times duration — so exposure limits are built on 85 dBA for 8 hours and a 3 dB exchange rate.
Why it matters in the exam
Noise-induced loss is sensorineural, usually bilateral and symmetrical, with a notch at 3000–6000 Hz and improvement at 8000. A flat loss is not typical of long occupational noise — it fits acute acoustic trauma (a blast) or another cause better. In a blast (tens to hundreds of dB SPL) the eardrum can rupture and the ossicles be damaged, the injury is often asymmetric (the ear facing the blast suffers more), and part of the loss recovers over hours to weeks while part remains. At the same level, broadband noise and mid-to-high-frequency noise are more harmful — partly because of canal resonance. OAEs fall before threshold changes, so they are a sensitive monitoring tool. Facial nerve palsy abolishes the stapedius reflex and can increase damage on that side. In animal studies, earlier moderate exposure "toughens" the ear (conditioning).
Limits: NIOSH sets 85 dBA as the 8-hour level and halves the time for each added 3 dB — so 4 hours at 88, 2 at 91, one at 94, 30 minutes at 97 and 15 minutes at 100 dBA; the Israeli regulations also set 85 dBA for 8 hours. US OSHA uses 90 dBA and a 5 dB exchange rate, and calls it a standard threshold shift (STS) when the mean of 2000, 3000 and 4000 Hz has worsened by at least 10 dB since baseline. Measurements use dBA (mimics the ear at moderate levels and discounts the lows); dBC is nearly flat and is used for loud noise and for choosing protectors. Noise maps are used at work to identify dangerous zones. Protectors: foam plugs give about 30 dB in the lab, but the NRR is measured under ideal conditions and must be derated in practice; ordinary plugs attenuate highs more than lows, so musicians use flat-attenuation plugs.
Key points
- Notch at 3000–6000, recovery at 8000; basal outer cells.
- NIOSH and Israel: 85 dBA over 8 hours; NIOSH halves time per 3 dB.
- OSHA: 90 and 5; STS — 10 dB at 2000–4000.
- dBA for measurement; OAE for early monitoring.
- The NRR needs derating; a flat plug for musicians.
Summary table
| Topic | The fact | For the exam |
|---|---|---|
| Audiometric pattern | Notch at 3000–6000 Hz, recovery at 8000 | Bilateral; flat — not typical of long exposure |
| Mechanism | Basal outer cells, around 4000 Hz | OAEs fall before threshold |
| Blast | Eardrum, ossicles, cochlea; often asymmetric | Part temporary, part permanent |
| NIOSH | 85 dBA over an 8-hour day; time halves per 3 dB | 100 dBA — 15 minutes; 94 — one hour |
| OSHA and STS | 90 dBA, exchange rate 5; STS — 10 dB at 2000–4000 | Compared with the baseline test |
| Weightings | dBA mimics the ear; dBC nearly flat | Occupational noise measured in dBA |
| Protectors | Foam about 30 dB in the lab; the NRR needs derating | For musicians — flat attenuation |
A free exam question
With long-term noise exposure, where will the main damage be?
- A. In the outer hair cells
- B. In the inner hair cells
- C. In the auditory nerve (VIII) fibres
- D. In the supporting cells
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Sources
- NIOSH, Criteria for a Recommended Standard: Occupational Noise Exposure, 1998 — Revised criteria (85 dBA, 3-dB exchange rate)
- OSHA 29 CFR 1910.95, 1983 — Occupational noise exposure; hearing conservation amendment (standard threshold shift)
- Israel, Safety at Work Regulations (Occupational Hygiene and Public Health of Workers Exposed to Noise), 1984 — Noise exposure limits and hearing tests
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.