This part of IEC 60318 describes a mechanical coupler for measurement of the output force of bone vibrators. The mechanical impedance of the coupler is specified in the frequency range 125 Hz to 8 000 Hz. The coupler is intended for calibration of audiometers using bone vibrators having a plane circular tip area of 175 mm² ± 25 mm² and for determining the performance of bone conduction hearing aids. The vibratory force developed by a bone vibrator is not, in general, the same on the coupler as on a person's mastoid. However, the IEC recommends its use as a means for the calibration of specified vibrators used in audiometry and for the exchange of specifications and of data on bone conduction hearing aids. NOTE Some bone vibrators of hearing aids and some non-standardised bone vibrators still used in audiometry have a curved surface. Users should be aware that those vibrators in general will not be loaded on the mechanical coupler with the same mechanical impedance as the one specified in Table 1 of this standard.