Overview
EN ISO 389-4:1998 (ISO 389-4:1994) defines reference levels for narrow-band masking noise used in the calibration of audiometric equipment. It specifies how much narrow-band noise (presented by air conduction from an earphone) must be added to the reference equivalent threshold sound pressure levels (RETSPLs) for corresponding pure-tone frequencies in pure‑tone audiometry. Data are provided for noise bandwidths of one‑third and one‑half octaves, and reference values are given as levels to add to RETSPLs when the masking earphone is placed on the specified coupler, ear simulator or artificial ear.
Key topics and technical requirements
- Scope: Calibration reference levels for narrow‑band masking noise in pure‑tone audiometers (air conduction).
- Reference presentation: Levels are quoted as additions to RETSPLs for the corresponding pure‑tone frequencies (as per ISO 389 series).
- Bandwidths covered: Preferred one‑third octave and additional intermediate frequencies; guidance also for bandwidths between one‑third and one‑half octave by interpolation.
- Effective masking level: Masking levels are calibrated in terms of the effective masking level (IEC convention) with a constant noise power spectral density across the passband.
- Baseline and calculation:
- If the noise bandwidth is less than the critical bandwidth at that centre frequency, the reference addition AL = 4 dB.
- If the noise bandwidth exceeds the critical bandwidth, AL is calculated by: AL = 4 dB + 10 log10(B / BC(f)), where B is the noise bandwidth and BC(f) is the critical bandwidth at frequency f. (Annex A describes derivation and critical bandwidth values.)
- Practical considerations: Notes on ipsilateral vs contralateral masking, skull attenuation and occlusion effect when masking is applied to the non‑test ear are included and should be considered in clinical application.
Applications and who uses it
- Audiologists and clinical audiology departments calibrating audiometers and setting masking protocols.
- Hearing test and calibration laboratories establishing traceable RETSPL-based masking levels.
- Manufacturers of audiometers, earphones, ear simulators and hearing testing equipment implementing standard reference levels.
- Occupational health and hearing conservation programs that require standardized masking procedures.
- Biomedical engineers and metrology specialists performing device verification and compliance testing.
Related standards
- ISO 389 series (reference RETSPLs for pure tones and other parts: insert earphones, bone vibrators, high-frequency tones)
- IEC standards for couplers and ear simulators: IEC 126, IEC 303, IEC 711, IEC 318
- IEC 645-1 (audiometer requirements and masking bandwidth conventions)
EN ISO 389-4 is essential for consistent, repeatable audiometric calibration, ensuring masking noise levels are referenced to international RETSPLs and enabling reliable pure‑tone hearing assessments.
