This part of ISO 389 specifies the following data applicable to the calibration of bone vibrators for puretone bone-conduction audiometry. Guidance on the practical application of this part of ISO 389 in the calibration of audiometers is given in Annex B. a) Reference equivalent threshold vibratory force levels (RETVFLs), corresponding to the threshold of hearing of young otologically normal persons by bone-conduction audiometry. RETVFL is the vibratory force level transmitted to a mechanical coupler of specified characteristics (see 5.4) by a vibrator when applied to the mechanical coupler under stated conditions of test and when energized at the voltage level corresponding to the normal threshold of hearing for location on the mastoid prominence. NOTE 1 Values for the differences in reference equivalent threshold vibratory force levels between location on the forehead and mastoid are included for information in Annex C. b) Essential characteristics of the bone vibrator and the method of coupling to the test subject, and to the mechanical coupler. c) Essential characteristics and the baseline masking noise level applied to the ear not under test. NOTE 2 Recommended procedures for carrying out bone-conduction audiometry are specified in ISO 8253‑1.