Overview - ISO 226:2023 (Acoustics - Normal equal-loudness-level contours)
ISO 226:2023 specifies the normal equal-loudness-level contours for pure tones - i.e., the combinations of frequency and sound pressure level (SPL) that are perceived as equally loud by human listeners. The standard gives both graphical (Annex A) and numerical (Annex B) representations for the preferred one‑third‑octave frequencies from 20 Hz to 12 500 Hz (in accordance with ISO 266). The data, derivation formulas and parameter table (Table 1) are based on binaural, free‑field, frontal‑incidence listening by otologically normal persons aged 18–25 years.
Key technical topics and requirements
- Scope and listening conditions: Free progressive plane wave, sound source directly in front of the listener, pure continuous tones, SPL measured at the head center position in absence of the listener, binaural listening, listeners 18–25 years with normal otological status.
- Frequency range and resolution: Preferred one‑third‑octave series from 20 Hz to 12 500 Hz (per ISO 266).
- Data presentation: Graphical contours (Annex A) and numerical tables for SPL vs loudness level (Annex B).
- Formulas: Informative formulas to convert between loudness level (phon) and sound pressure level (dB) (Formulas (1) and (2)), with frequency‑dependent parameters listed in Table 1.
- Validity limits: Formulas apply reliably for loudness levels from 20 phon up to specified upper limits (20 Hz–4 kHz: 90 phon; 5 kHz–12.5 kHz: 80 phon). Values below 20 phon and certain very high loudness levels are informative only due to limited experimental data.
- Alignment and updates: Revision aligns with ISO 389‑7 for threshold data and corrects prior systematic errors (changes up to ~0.6 dB versus the 2003 edition).
Practical applications - who uses ISO 226:2023
- Acoustic and audio engineers for loudness calibration, equalization, and perceptual audio design.
- Psychoacoustics researchers studying human loudness perception and masking.
- Audiologists and clinical calibration laboratories referencing normative loudness relationships and audiometric calibration.
- Manufacturers of headphones, loudspeakers and hearing devices using standard loudness contours for product evaluation and tuning.
- Environmental noise assessors, signal processing and telecommunication engineers for perceptual weighting and loudness‑related metrics.
Related standards
- ISO 266 - Preferred frequencies (one‑third‑octave series)
- ISO 389‑7 - Reference zero values / hearing threshold (free‑field) - used for threshold alignment
- ISO 532‑1 / ISO 532‑2 and ECMA‑418 - other loudness calculation standards (for arbitrary or tonal loudness)
ISO 226:2023 is an essential reference for anyone needing standardized, validated equal‑loudness data and the formulas to convert between perceptual loudness (phon) and physical sound pressure level (dB).