This part of IEC 60318 specifies an acoustic coupler for the measurement of supra- aural audiometric earphones in the frequency range from 125 Hz to 8 000 Hz. The sound pressure developed by an earphone is not, in general, the same in the coupler as in a person's ear. However, the acoustic coupler can be used as an objective and reproducible means of measuring the output of supra-aural earphones. It can be used for specif ying reference equivalent threshold sound pressure levels (RETSPL) for the calibration of audiometers.