Source: https://jmrplens.github.io/phonometry/guides/sections/hearing-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)](/phonometry/guides/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)](/phonometry/guides/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)](/phonometry/guides/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](/phonometry/guides/sections/speech/). ## Pages in this section - [Hearing threshold (age and reference zero)](/phonometry/guides/hearing-threshold/): the ISO 7029:2017 age-related threshold distribution and the ISO 389-7:2005 reference threshold of hearing. - [Noise-induced hearing loss (ISO 1999)](/phonometry/guides/noise-induced-hearing-loss/): NIPTS and its population distribution, and the combination with age into HTLAN. - [Occupational Noise Exposure (ISO 9612)](/phonometry/guides/occupational-exposure/): the three measurement strategies for LEX,8h with the Annex C uncertainty budget.