Source: https://docs.jmrp.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://docs.jmrp.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. [Audiometric Test Methods (ISO 8253-1 and -2)](https://docs.jmrp.io/phonometry/perception/audiometry/) says what it takes to measure a threshold against that zero: a test room quiet enough (ISO 8253-1 Clause 11) or with a qualified sound field (ISO 8253-2), the rules that turn responses into a hearing threshold level, and the uncertainty of that level. [Occupational Noise Exposure (ISO 9612)](https://docs.jmrp.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://docs.jmrp.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://docs.jmrp.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://docs.jmrp.io/phonometry/perception/hearing-threshold/): the ISO 7029:2017 age-related threshold distribution and the ISO 389-7:2005 reference threshold of hearing. - **Measurement**: [Audiometric Test Methods (ISO 8253-1 and -2)](https://docs.jmrp.io/phonometry/perception/audiometry/): the test room's ambient noise and sound field, the ascending, bracketing, automatic and sweep-frequency threshold rules, and the Annex A uncertainty. - **Exposure**: [Occupational Noise Exposure (ISO 9612)](https://docs.jmrp.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://docs.jmrp.io/phonometry/perception/noise-induced-hearing-loss/): NIPTS and its population distribution, and the combination with age into HTLAN. - **Protection**: [Hearing Protectors (ISO 4869-1, -2, -3 and -6)](https://docs.jmrp.io/phonometry/perception/hearing-protectors/): the attenuation measured on sixteen subjects with its uncertainty and its significance test, the octave-band, HML and SNR methods that say what a protector actually leaves at the ear, what an active noise reduction earmuff adds, and the insertion loss that screens an earmuff on a test fixture. ## 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 audiometer's hardware is driven here. Nothing generates a test tone or presents it to a subject: ISO 8253's rules turn the responses a tester records into a threshold, and judge the room it was measured in, but the responses themselves are data, and so are the threshold levels the other pages consume and produce.