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phonometry

Psychoacoustics

Psychoacoustics replaces the question “how many decibels?” with “what does the listener perceive?”. Its base quantity is loudness: a perceptual magnitude in sones, computed by auditory models that account for the ear’s filtering, masking and compression. On top of loudness sit the sound quality sensations that distinguish two equally loud sounds: sharpness (high-frequency emphasis), tonality (audible discrete tones), roughness (fast modulation) and fluctuation strength (slow modulation). And on top sits a combined annoyance metric that weighs loudness, sharpness, roughness and fluctuation strength into a single scalar.

Every metric here is a magnitude fixed by a reference sound rather than by a physical unit, and knowing the anchor is what makes a number readable: 1 sone is a 1 kHz tone at 40 dB SPL, 1 acum a critical-band-wide noise at 1 kHz and 60 dB, 1 asper a 1 kHz carrier fully modulated at 70 Hz at 60 dB, 1 vacil the same carrier modulated at 4 Hz, and 1 tu_HMS a 1 kHz tone at 40 dB. They are tabulated together, beside the speech and hearing scales, under “Reading the numbers” on the section overview.

The two families of pages differ in purpose, and that difference decides what you can conclude. Loudness, sharpness, roughness and fluctuation strength are open-ended magnitudes for comparing designs: there is no pass mark, and the useful statement is always a comparison. The two tonal pages end in a verdict against a criterion, because they exist to justify a declaration or a penalty. ECMA-418-2 sits between the two: it attaches informative prominence criteria to its tonality (0.4 tu_HMS on a band), roughness (0.2 asper) and fluctuation strength (0.2 vacil_HMS), which is the closest thing to a pass mark in the magnitude family. All of them share one prerequisite: an absolutely calibrated signal in pascals, because every metric here is level-dependent.

Loudness is the foundation page: the Zwicker reference method of ISO 532-1 with its one-page fiche, together with the ISO 226:2023 equal-loudness contours that anchor the perceptual scale for pure tones. The newer model families, Moore-Glasberg per ISO 532-2/-3 and the Sottek Hearing Model of ECMA-418-2, continue in Advanced Loudness, which also carries the model-choice table. Sound Quality Metrics adds sharpness per DIN 45692 and the ECMA-418-2 tonality, roughness and fluctuation strength that share the Sottek front-end.

Tones in noise get two dedicated pages because two different questions are asked of them. Prominent Discrete Tones (ECMA-418-1) answers a product-noise question: is this tone prominent by the tone-to-noise and prominence-ratio criteria used in IT-equipment declarations? Objective audibility of tones in noise (ISO/PAS 20065) answers an environmental one: by how many decibels does the tone exceed its masking threshold, the audibility that feeds the tonal penalty of ISO 1996-2.

Psychoacoustic annoyance and fluctuation strength closes the chain with the Fastl & Zwicker model, which combines loudness, sharpness, roughness and the slow-modulation sensation of fluctuation strength into a single annoyance value. Read it last: three of its four inputs come from the earlier pages, and it supplies the fourth, fluctuation strength, itself, in both the Fastl & Zwicker closed form and the Osses 2016 signal model. The ECMA-418-2 fluctuation strength on the Sound Quality page is a further, normative model of the same sensation, under a different unit name.

Pages in this section

Section titled “Pages in this section”

What this section does not cover

Section titled “What this section does not cover”

  • Everything here is monaural. The binaural combinations ECMA-418-2 defines for loudness, roughness and fluctuation strength are not implemented, so a stereo or binaural recording is analysed one channel at a time, and no model here accounts for localisation or for spatial release from masking. Two optional refinements are also left out: the entropy weighting of clause 7.1.6, which needs an external rotational-speed signal, and the small adjustment footnote 47 permits.

  • A verdict is never complete. The prominent flag the tone-prominence functions return is the numeric criterion alone; ECMA-418-1 also requires aural confirmation and a lower-threshold-of-hearing screen, both of which stay with the caller. The tone-audibility module is weighting-agnostic and does not apply the A-weighting clause 5.3.2 requires, so A-weight the spectrum before passing it, and it takes an already-computed narrow-band spectrum rather than building one from a recording.

  • Two documented deviations are worth knowing. ISO 532-3 prescribes resampling to 32 kHz before the running FFT; this implementation works at the native rate, a deviation that stays inside the standard’s expanded uncertainty but that you should undo by resampling first if strict clause-by-clause conformance matters. And the Osses 2016 fluctuation-strength signal model is validated for amplitude-modulated stimuli only, with a documented floor — a steady 1 kHz tone reads about 0.09 vacil rather than 0.

  • Finally, none of these metrics is a community response: annoyance here is a laboratory sensation computed from a signal, while the annoyance a neighbourhood reports is a social-survey quantity handled through the indicators of Environment and transport.

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