---
title: "Hearing and perception"
description: "How listeners perceive sound and what noise does to hearing: psychoacoustic sensations (loudness, sharpness, roughness, tonality, annoyance), speech intelligibility (STI and SII), and the statistics of hearing thresholds, hearing damage and occupational exposure."
---

A sound pressure level says how much sound there is; this section is about
what a **listener** makes of it. Its three subsections answer three different
questions. **Psychoacoustics** quantifies sensations: how loud a sound is
perceived to be, how sharp, rough or tonal it is, and how those sensations
combine into annoyance. **Speech** asks how well spoken words survive the trip
from a talker to a listener, through a room, a sound system or background
noise. And **hearing and exposure** covers the ear itself: where the hearing
threshold sits across age and population, how noise permanently shifts it, and
how a working day's exposure is measured and reported.

The three build on each other in one direction: the psychoacoustic models
consume calibrated signals or band spectra from the
[core analysis](/phonometry/guides/sections/core-signal-analysis/); the speech
indices consume band levels and, in the SII's case, the hearing thresholds
that the exposure pages quantify; and the exposure metrics feed the
hearing-damage model of ISO 1999.

A good entry point is [Loudness](/phonometry/guides/loudness/): it introduces
the perceptual scale (the sone) and the auditory models that most other
metrics in this section reuse or extend.

## [Psychoacoustics](/phonometry/guides/sections/psychoacoustics/)

The perceptual sensations of sound: loudness and the metrics layered on it.

- [Loudness](/phonometry/guides/loudness/): Zwicker (ISO 532-1),
  Moore-Glasberg (ISO 532-2/3) and Sottek (ECMA-418-2) loudness in sones,
  plus the ISO 226:2023 equal-loudness contours.
- [Sound Quality Metrics](/phonometry/guides/sound-quality/): sharpness
  (DIN 45692) and the ECMA-418-2 Sottek Hearing Model tonality, roughness
  and fluctuation strength.
- [Prominent Discrete Tones (ECMA-418-1)](/phonometry/guides/tone-prominence/):
  the tone-to-noise and prominence ratios that decide whether a discrete tone
  is prominent.
- [Objective audibility of tones in noise (ISO/PAS 20065)](/phonometry/guides/tone-audibility/):
  the engineering method for the audibility of a tone above the masking
  threshold, feeding the ISO 1996-2 tonal adjustment.
- [Psychoacoustic annoyance and fluctuation strength](/phonometry/guides/psychoacoustic-annoyance/):
  the Fastl & Zwicker annoyance model combining loudness, sharpness, roughness
  and fluctuation strength.

## [Speech](/phonometry/guides/sections/speech/)

Two complementary indices of speech intelligibility: STI rates the
transmission channel, SII rates the listening condition.

- [Speech Transmission Index (STI)](/phonometry/guides/speech-transmission/):
  the IEC 60268-16 modulation transfer function, the indirect method from an
  impulse response, and direct STIPA measurement.
- [Speech Intelligibility Index](/phonometry/guides/speech-intelligibility/):
  the ANSI S3.5-1997 one-third-octave-band SII from speech, noise and hearing
  threshold spectra.

## [Hearing and exposure](/phonometry/guides/sections/hearing-exposure/)

The hearing threshold, what noise does to it, and how exposure is measured.

- [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/):
  the noise-induced permanent threshold shift and its combination with age
  into HTLAN.
- [Occupational Noise Exposure (ISO 9612)](/phonometry/guides/occupational-exposure/):
  the task-based, job-based and full-day strategies for LEX,8h with the
  Annex C uncertainty budget.
