Overview
ISO 8253-2:2009 - Acoustics: Audiometric test methods - Part 2 defines requirements and procedures for sound field audiometry using pure tones, frequency‑modulated (FM) tones, and narrow‑band test signals presented via one or more loudspeakers. Its primary purpose is determination of hearing threshold levels in the frequency range 125 Hz to 8 000 Hz (extendable to 20 Hz–16 000 Hz). The standard specifies test signal characteristics, requirements for free, diffuse, and quasi‑free sound fields, calibration and reporting conventions, and procedures to maximize accuracy and reproducibility of sound‑field hearing tests. It excludes hand‑held loudspeakers and speech test signals.
Key Topics and Requirements
- Test signals
- Allowed signals: pure tones (in appropriate fields), FM tones, and narrow‑band noise.
- Pure tones: used only in a free sound field to avoid standing‑wave errors.
- FM tones: defined by carrier frequency, modulating waveform (sinusoidal or triangular), repetition rate and frequency deviation - these parameters must be reported.
- Narrow‑band noise: centre frequency and bandwidth must comply with IEC 60645‑1 masking noise specifications.
- Sound fields
- Free sound field: negligible room boundary effects (required for pure tones).
- Diffused field: statistically uniform energy density and random propagation directions.
- Quasi‑free field: moderate boundary effects with specified tolerances.
- Frequency range & tolerances
- Nominal audiometric frequencies per IEC 60645‑1 / ISO 266; frequency tolerances and FM modulation tolerances are defined for reproducibility.
- Room and equipment
- Ambient noise limits in the test room, calibration methods, and scales for measurement are addressed.
- Maintenance and calibration regimes (routine listening checks to periodic electroacoustic and basic calibration tests) are specified.
- Procedures & reporting
- Methods for threshold determination (monaural, binaural), testing with hearing aids (functional gain), screening audiometry, and standardized reporting (including graphical displays, Annex A).
Applications - Who Uses It
- Audiologists and clinical hearing professionals for clinical sound‑field testing (especially pediatric assessments and aided listening tests).
- Hearing‑aid manufacturers and dispensers to measure functional gain in the sound field.
- Acoustic engineers and test‑room designers ensuring test environments meet free/diffuse field criteria.
- Research labs and regulatory bodies applying standardized audiometric methods for reproducible results.
Practical benefits include improved accuracy, repeatability of thresholds, and consistent measurement of hearing‑aid performance in real listening environments.
Related Standards (normative references)
Keywords: ISO 8253-2:2009, sound field audiometry, audiometric test methods, pure-tone, FM tones, narrow-band noise, hearing threshold, calibration, free field, diffuse field, functional gain.