This part of IEC 645 specifies requirements for audiometers or parts thereof designed to provide a means of presenting speech sounds to a subject in a standardized manner, for example for the measurement of speech recognition. Audiometers are classified into four types: type A instruments provide a wide range of facilities, while type B provides only basic facilities. For both types, the audiometer may be calibrated in terms of a free-field equivalent output level of the earphones and then be des­ ignated as type A-E or B-E. This standard excludes the requirements for speech audiometers provided with frequency response adjustments intended to determine an optimal intelligibility frequency response curve for each subject. The purpose of this standard is to ensure that tests of hearing using speech as the test stimulus, on a given human ear performed with different audiometers which comply with this standard, shall give substantially the same results for a given calibration method. Two methods of specification, calibration and testing are given for the output levels generated by the earphones and the bone vibrator: a) a free-field equivalent output level method with audiometers of types A-E and B-E; b) an uncorrected coupler output level method with audiometers of types A and B. NOTE - The results will also depend on factors other than the audiometer for example test material, test room, speakers, etc.