Source: https://jmrplens.github.io/phonometry/perception/hearing/ # Hearing and exposure This section connects three quantities that regulation treats as one story: where a population's **hearing threshold** sits, how occupational **noise exposure** is measured, and how that exposure **shifts the threshold** permanently over a working life. [Hearing threshold (age and reference zero)](https://jmrplens.github.io/phonometry/perception/hearing-threshold/) establishes the baseline. **ISO 7029:2017** gives the statistical distribution of hearing threshold with age for an otologically normal population: the slow, high-frequency-first loss of presbycusis, resolved by age, sex and population fractile. **ISO 389-7:2005** fixes the other end of the scale: the reference threshold of hearing, the physical sound pressure level that audiometric 0 dB HL corresponds to under free-field and diffuse-field listening. [Occupational Noise Exposure (ISO 9612)](https://jmrplens.github.io/phonometry/perception/occupational-exposure/) measures the cause. A regulated daily exposure level LEX,8h is assembled from samples of a real working day by one of three strategies (task-based, job-based or full-day), and reported with the normative Annex C uncertainty budget and its one-sided 95 % upper limit, which is what an occupational hygienist actually compares against action values. [Noise-induced hearing loss (ISO 1999)](https://jmrplens.github.io/phonometry/perception/noise-induced-hearing-loss/) predicts the effect. From the exposure level and duration it gives the noise-induced permanent threshold shift (NIPTS) with its population spread, concentrated at the characteristic 4 kHz notch, and combines it with the ISO 7029 age component into the hearing threshold level associated with age and noise (HTLAN): the quantity used to estimate hearing handicap and compensation risk in an exposed population. Read the threshold page first, then ISO 9612, then ISO 1999: baseline, exposure, damage. That way the pipeline is explicit: ISO 9612 produces the LEX,8h that ISO 1999 consumes, and ISO 1999 builds on the ISO 7029 statistics. The perceptual consequences of a shifted threshold, such as reduced speech intelligibility, are picked up by the SII in the [Speech section](https://jmrplens.github.io/phonometry/speech/). **Three pages, three different decibels**, and keeping them apart is most of the work. A **hearing threshold level** is in dB HL, measured relative to the audiometric zero, so 0 dB HL is a *different* sound pressure at every frequency — exactly what ISO 389-7 tabulates. A **daily exposure level** is in A-weighted decibels normalised to eight hours: an energy dose of the sound outside the ear, with no listener in it. A **threshold shift** is a difference of two dB HL values, so it may be added to a hearing level and never to a sound pressure level. The chain between them runs one way only: ISO 9612 delivers a single A-weighted LEX,8h into the ISO 1999 formulae, which return dB HL. The only bridge back from hearing level to physical sound pressure is the ISO 389-7 reference threshold on the threshold page — which is also what the SII needs when a raised threshold is used as an input. ## Pages in this section In the order the chain runs. - **Baseline** — [Hearing threshold (age and reference zero)](https://jmrplens.github.io/phonometry/perception/hearing-threshold/): the ISO 7029:2017 age-related threshold distribution and the ISO 389-7:2005 reference threshold of hearing. - **Exposure** — [Occupational Noise Exposure (ISO 9612)](https://jmrplens.github.io/phonometry/perception/occupational-exposure/): the three measurement strategies for LEX,8h with the Annex C uncertainty budget. - **Damage** — [Noise-induced hearing loss (ISO 1999)](https://jmrplens.github.io/phonometry/perception/noise-induced-hearing-loss/): NIPTS and its population distribution, and the combination with age into HTLAN. ## What this section does not cover **Nothing here is a verdict about a person.** ISO 1999 does not define a hearing handicap or a compensable fence — that line is set by national regulation, and the library applies none of it, so you supply and check the criterion yourself. The exposure side stops one step later: the LEX,8h and its one-sided 95 % upper limit come out of ISO 9612, and the fiche written by `ExposureResult.report()` does compare the LEX,8h against the Directive 2003/10/EC action and limit values (80, 85 and 87 dB(A)) and issues the PASS/FAIL verdict against the limit value — but that is a verdict on an unprotected exposure, not on a person: the effective exposure behind hearing protectors, and any stricter national transposition, are still yours to check. Two implementation boundaries follow the standards. Only **database A** is implemented for ISO 1999: `htlan` always draws its age component from ISO 7029:2017, and substituting a nationally measured control population (clauses 6.2.3 and 6.2.4) means computing that database elsewhere and passing it in. Of ISO 389-7, only the Table 1 reference values are implemented, not the procedures by which they were established. And no audiometry happens here. Nothing generates a test tone, drives an audiometer or corrects for an earphone coupler: the pages consume and produce threshold levels as data.