Source: https://jmrplens.github.io/phonometry/perception/ # Hearing and perception 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](https://jmrplens.github.io/phonometry/signals/); the speech indices consume band levels and, in the SII's case, the hearing thresholds that the hearing pages quantify; and the exposure metrics feed the hearing-damage model of ISO 1999. A good entry point is [Loudness](https://jmrplens.github.io/phonometry/psychoacoustics/loudness/): it introduces the perceptual scale (the sone) and the auditory models that most other metrics in this section reuse or extend. The derivations behind these methods — the critical-band and excitation-pattern models, the masking formulations and the modulation-transfer chain — are gathered on the [perception theory page](https://jmrplens.github.io/phonometry/reference/theory/perception/), which the individual guides cite clause by clause. ### Reading the numbers Almost every perceptual scale in this section is defined by a **reference sound** rather than by a physical unit, so the first thing to learn about each is its anchor: the sound that reads exactly 1. | Quantity | Unit | The sound that reads 1 | Criterion? | Page | |---|---|---|---|---| | Loudness | sone | 1 kHz tone at 40 dB SPL (also 40 phon) | none | [Loudness](https://jmrplens.github.io/phonometry/psychoacoustics/loudness/) | | Sharpness | acum | critical-band-wide noise at 1 kHz, 60 dB SPL | none | [Sound Quality Metrics](https://jmrplens.github.io/phonometry/psychoacoustics/sound-quality/) | | Roughness | asper | 1 kHz tone at 60 dB, fully modulated at 70 Hz | 0.2 asper (informative) | [Sound Quality Metrics](https://jmrplens.github.io/phonometry/psychoacoustics/sound-quality/) | | Fluctuation strength | vacil | the same carrier modulated at 4 Hz | 0.2 vacil (informative) | [Sound Quality Metrics](https://jmrplens.github.io/phonometry/psychoacoustics/sound-quality/) | | Tonality | tu_HMS | 1 kHz tone at 40 dB SPL | 0.4 tu_HMS (informative) | [Sound Quality Metrics](https://jmrplens.github.io/phonometry/psychoacoustics/sound-quality/) | | Tone audibility | dB | — (a level difference above masking) | ISO 1996-2 adjustment | [Tone audibility](https://jmrplens.github.io/phonometry/psychoacoustics/tone-audibility/) | | STI | 0 to 1 | — | Annex F letters, U to A+ | [Speech Transmission Index](https://jmrplens.github.io/phonometry/speech/speech-transmission/) | | SII | 0 to 1 | — | none standardised | [Speech Intelligibility Index](https://jmrplens.github.io/phonometry/speech/speech-intelligibility/) | | Threshold shift | dB HL | — (a difference of two hearing levels) | ISO 1999 statistics | [Noise-induced hearing loss](https://jmrplens.github.io/phonometry/hearing/noise-induced-hearing-loss/) | Loudness, sharpness, roughness and fluctuation strength are **ratio scales with no pass/fail line**: twice the number means twice the sensation, so a 20-sone appliance is heard as about twice as loud as a 10-sone one, which is why appliance declarations set limits in sones rather than in decibels. The tonal metrics and the speech indices do carry criteria, which is why the tone pages end in a verdict and the loudness pages do not. The three speech numbers all live in [0, 1] and are **not the same number**. An STI of 0.6 falls in Annex F band D, typical of a good lecture room; an SII of 0.6 means roughly 60 % of the importance-weighted speech spectrum is audible; and a STOI of 0.6 has no absolute meaning at all, because the mapping from index to words understood is fitted per listening-test corpus, so STOI is only ever read as a difference between two processors on the same material. Never substitute one for another in a specification, and when a requirement quotes a number, check which standard it belongs to before computing anything. ## [Psychoacoustics](https://jmrplens.github.io/phonometry/psychoacoustics/) The perceptual sensations of sound: loudness and the metrics layered on it. - [Loudness](https://jmrplens.github.io/phonometry/psychoacoustics/loudness/): the ISO 532-1 Zwicker loudness in sones, plus the ISO 226:2023 equal-loudness contours. - [Advanced Loudness (ISO 532-2/-3, ECMA-418-2)](https://jmrplens.github.io/phonometry/psychoacoustics/advanced-loudness/): the Moore-Glasberg stationary and time-varying methods and the Sottek Hearing Model loudness, with the model-choice table. - [Sound Quality Metrics](https://jmrplens.github.io/phonometry/psychoacoustics/sound-quality/): sharpness (DIN 45692) and the ECMA-418-2 Sottek Hearing Model tonality, roughness and fluctuation strength. - [Prominent Discrete Tones (ECMA-418-1)](https://jmrplens.github.io/phonometry/psychoacoustics/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)](https://jmrplens.github.io/phonometry/psychoacoustics/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](https://jmrplens.github.io/phonometry/psychoacoustics/psychoacoustic-annoyance/): the Fastl & Zwicker annoyance model combining loudness, sharpness, roughness and fluctuation strength. ## [Speech](https://jmrplens.github.io/phonometry/speech/) Two complementary indices of speech intelligibility, STI for the transmission channel and SII for the listening condition, plus the signal-based STOI and ESTOI measures. - [Speech Transmission Index (STI)](https://jmrplens.github.io/phonometry/speech/speech-transmission/): the IEC 60268-16 modulation transfer function, the indirect method from an impulse response, and direct STIPA measurement. - [Speech Intelligibility Index](https://jmrplens.github.io/phonometry/speech/speech-intelligibility/): the ANSI S3.5-1997 one-third-octave-band SII from speech, noise and hearing threshold spectra. - [Objective Intelligibility (STOI & ESTOI)](https://jmrplens.github.io/phonometry/speech/objective-intelligibility/): the correlation-based measures for time-frequency weighted noisy speech, from a clean/degraded pair. ## [Hearing and exposure](https://jmrplens.github.io/phonometry/hearing/) The hearing threshold, what noise does to it, and how exposure is measured. - [Hearing threshold (age and reference zero)](https://jmrplens.github.io/phonometry/hearing/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)](https://jmrplens.github.io/phonometry/hearing/noise-induced-hearing-loss/): the noise-induced permanent threshold shift and its combination with age into HTLAN. - [Occupational Noise Exposure (ISO 9612)](https://jmrplens.github.io/phonometry/hearing/occupational-exposure/): the task-based, job-based and full-day strategies for LEX,8h with the Annex C uncertainty budget. ## What this section does not cover **No listener is tested here, and no verdict about a person is issued.** Every model on these pages predicts what a population, or a standard listener, would perceive from a calibrated signal: none of them runs an audiometric session, none diagnoses a hearing loss, and ISO 1999 explicitly declines to define a hearing handicap or a compensable fence — that line is national regulation, and nothing here applies one. Every prominence and audibility verdict is likewise the numeric criterion only: ECMA-418-1 also requires aural confirmation of a prominent tone, and that stays with you. **Everything is monaural.** The binaural combinations of ECMA-418-2 are not implemented, so a two-channel recording is analysed one ear at a time, and nothing here models localisation, spatial release from masking or binaural loudness summation. **No listening test is replaced.** STOI returns the correlation-based index and not a percentage of words understood, because that mapping is fitted per listening-test corpus; the SII returns an audibility fraction and not a score; and no page predicts annoyance in a community, which is a social-survey quantity rather than a psychoacoustic one — the community indicators are [Environment and transport](https://jmrplens.github.io/phonometry/environment/). Finally, these models start from a **calibrated** signal or spectrum in pascals, because every one of them is level-dependent. Feeding them raw soundcard samples produces a number with an arbitrary reference, which is a different failure from a wrong answer: it looks plausible. ## Before and after these pages Every model here consumes a calibrated signal or a calibrated spectrum, so the calibration and weighting that produce one are in [Signal analysis](https://jmrplens.github.io/phonometry/signals/), and [Build a sound level meter](https://jmrplens.github.io/phonometry/signals/sound-level-meter/) runs that chain end to end on one runnable page. The derivations are in [Perception and hearing theory](https://jmrplens.github.io/phonometry/reference/theory/perception/), from the equal-loudness contours to the modulation transfer function. If you arrived here from a search and want the shape of the whole library, [What do you need to measure?](https://jmrplens.github.io/phonometry/start/tasks/) indexes it by the job and [All guides](https://jmrplens.github.io/phonometry/) lists every page with a line on each.