Overview
ISO 389-4:1994 - "Acoustics - Reference zero for the calibration of audiometric equipment - Part 4: Reference levels for narrow‑band masking noise" specifies standardized reference levels for narrow‑band masking noise used in pure‑tone audiometry. The standard gives additive levels (in dB) to be applied to the reference equivalent threshold sound pressure levels (RETSPL) for the corresponding pure‑tone frequencies when masking noise is presented by air conduction through earphones. Data are provided for one‑third‑octave and one‑half‑octave noise bandwidths; rules for interpolation and calculation are included.
Key topics and requirements
- Reference additions to RETSPL: Tabulated values (additive dB) for standard audiometric centre frequencies to quantify narrow‑band masking requirements.
- Bandwidth coverage: Values specified for both one‑third‑octave and one‑half‑octave masking noises; provisions for interpolating intermediate bandwidths.
- Measurement context: Levels apply when the masking earphone is placed on an acoustic coupler, ear simulator or artificial ear, per relevant IEC coupling standards.
- Psychoacoustic basis: Masking levels are derived from critical‑band and ipsilateral masking assumptions; Annex A documents the derivation and critical‑band widths used.
- Calculation guidance: The standard includes a formula for deriving masking level adjustments when noise bandwidth differs from a critical‑band assumption (e.g., AL = 4 dB + 10 log10[Q/c(f)] dB as given in the document).
- Practical considerations: Notes on skull attenuation (interaural transmission) and the occlusion effect when masking is delivered to the non‑test ear, which influence effective masking.
Applications and users
- Clinical audiology: Ensures consistent, repeatable masking when conducting pure‑tone threshold testing to prevent cross‑hearing.
- Audiometer manufacturers and calibrators: Use the reference levels to set and verify masking noise output during device calibration and type‑approval.
- Calibration laboratories and test houses: Apply ISO 389‑4 when measuring earphone masking output on couplers or ear simulators.
- Occupational and research audiometry: Standardizes masking procedures for clinical studies, hearing conservation programs, and regulatory testing.
Related standards
- ISO 389 series (RETSPL and related parts: ISO 389‑1/‑2/‑3)
- IEC standards for couplers and ear simulators (e.g., IEC 126, IEC 303, IEC 711, IEC 318)
- IEC 645‑1 (audiometers - tonal audiometers)
Keywords: ISO 389-4:1994, acoustics standard, audiometric equipment calibration, narrow‑band masking noise, masking levels, audiometer calibration, RETSPL, one‑third octave, half‑octave.
