---
title: "Hearing and exposure"
description: "The statistics of hearing and the noise that damages it: the ISO 7029 age-related threshold distribution and ISO 389-7 reference zero, the ISO 1999 noise-induced permanent threshold shift, and the ISO 9612 occupational-exposure survey."
---

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.
