EN ISO 389-9:2009 - "Acoustics - Reference zero for the calibration of audiometric equipment - Part 9: Preferred test conditions for the determination of reference hearing threshold levels" (ISO 389-9:2009) defines preferred test conditions used when measuring human hearing thresholds to establish standardized reference hearing threshold levels. Prepared by ISO/TC 43 and adopted by CEN, the document provides harmonized procedures for subject selection, test environment, equipment, transducer handling and test signals so that threshold data from different laboratories can be reliably collated.
Key topics and technical requirements
- Scope: Test conditions for producing threshold data used to set reference zero levels for audiometric calibration.
- Test subjects: Minimum of 25 subjects, both sexes, preferably equal numbers; age 18–25 years; otologically normal (no ear disease, no obstructing wax, no excessive noise/drug exposure or familial hearing loss). Tympanometry required; middle ear pressure must be within ±50 daPa before testing.
- Ambient noise: Acoustic environment must meet ISO 8253-1 limits to permit measurement down to −10 dB with a maximum +2 dB uncertainty from ambient noise. For typical supra-aural earphones, ambient noise should be at least 10 dB below reference values. One‑third‑octave band limits up to 16 kHz apply.
- Test procedure: Pure-tone thresholds measured per ISO 8253-1 (monaural) and ISO 8253-2 (binaural/sound field). Use bracketing or ascending methods; each subject receives at least one training session.
- Audiometric equipment & calibration: Equipment shall meet IEC/60645 series requirements. Regular calibration and maintenance per ISO 8253-1 clause 12; attention to stimulus gating and tone/attenuator switching artifacts.
- Transducers and fitting:
- Classes: supra-aural, circumaural, insert earphones, bone vibrators, and loudspeakers (sound field).
- Calibration couplers/simulators per IEC 60318 (parts 1,3,4,5,6).
- Headband force specification referencing 145 mm cushion separation; correct fitting procedures and positioning required.
- Test signals: Pure tones with maximum frequency deviation 0.1% and duration 1–2 s; switching must meet IEC timing requirements.
Applications and who uses it
- Audiology research laboratories conducting normative hearing studies.
- Calibration laboratories and manufacturers of audiometers and transducers establishing reference-zero levels.
- Clinical audiologists and occupational health services ensuring equipment calibration traceability.
- Standards bodies and test houses collating multi-site threshold data for new reference-level standards.
Related standards
Keywords: EN ISO 389-9:2009, reference hearing threshold levels, audiometric calibration, test conditions, audiometric equipment, ambient noise, transducer calibration.