ISO 389-3:2016

Overview

ISO 389-3:2016 defines the reference zero for bone-conduction audiometric equipment by specifying Reference Equivalent Threshold Vibratory Force Levels (RETVFL) for pure tones and bone vibrators. The standard ensures consistent calibration of bone vibrators so that bone‑conduction hearing thresholds measured on young, otologically normal people are compatible with air‑conduction reference zeros (ISO 389 series). It covers test conditions, vibrator and coupler characteristics, and required masking noise for reliable clinical and laboratory calibration.

Key Topics and Requirements

• RETVFL (Reference Equivalent Threshold Vibratory Force Levels): Mean vibratory force levels (in dB re 1 µN) corresponding to threshold hearing of 18–25 year old otologically normal subjects when a vibrator is applied to the mastoid prominence.
• Bone vibrator characteristics and fitting: Essential geometric and application-force specifications for bone vibrators and the method of coupling to the mastoid (and forehead - differences provided in Annex C).
• Mechanical coupler: Use of a calibrated mechanical coupler (see IEC 60318-6) to measure vibratory force levels under specified static force and impedance conditions.
• Test conditions and signals: Specified test signals and procedures for determining RETVFL values and for calibrating audiometers.
• Masking noise and baseline masking level: Defined masking noise characteristics and the baseline masking level to be applied to the non-test ear, including treatment of the occlusion effect (raising masking level to offset occlusion-induced threshold changes).
• Informative annexes:
  • Annex A: derivation of RETVFL values
  • Annex B: guidance for practical calibration of bone-conduction audiometers
  • Annex C: mastoid vs. forehead RETVFL differences
  • Annex D: thresholds below 250 Hz
• Normative references: Related calibration and test procedure standards such as ISO 389-1/-2/-4 and IEC 60318-6; recommended measurement procedures referenced in ISO 8253-1.

Applications and Users

Who uses ISO 389-3:

• Audiologists and clinical audiology departments for standardized bone‑conduction testing.
• Manufacturers of audiometers and bone vibrators for product design and calibration.
• Calibration laboratories and test houses verifying bone vibrator output against RETVFL.
• Regulatory bodies and test laboratories ensuring international compatibility of audiometric measurements.

Practical uses:

• Calibrating bone‑conduction transducers to an internationally recognized reference zero.
• Ensuring comparability between air‑ and bone‑conduction thresholds in diagnostic audiometry.
• Implementing correct masking protocols and coupler-based verification during device commissioning and maintenance.

Related Standards

• ISO 389-1, ISO 389-2, ISO 389-4 (other parts of the ISO 389 reference-zero series)
• IEC 60318-6 (mechanical coupler for bone vibrator measurement)
• ISO 8253-1 (recommended bone‑conduction audiometry procedures)

Keywords: ISO 389-3, RETVFL, bone vibrator calibration, bone-conduction audiometry, mechanical coupler, masking noise, occlusion effect, audiometric equipment calibration.