This part of IEC60118 gives recommendations for the measurement of the performance characteristics of air conduction hearing aids based on a free field technique and measured with an acoustic coupler. This part of IEC60118 is applicable to the measurement and evaluation of the electroacoustical characteristics of hearing aids, for example for type testing and manufacturer data sheets. The test results obtained by the methods specified in this part of IEC60118 will express the performance under conditions of the test and may deviate substantially from the performance of the hearing aid under actual conditions of use. This part of IEC60118 uses an acoustic coupler according to IEC60318‑5 which is only intended for loading a hearing aid with a specified acoustic impedance and is not intended to model the sound pressure in a person’s ear. The use of this acoustic coupler will yield different results from those obtained using the occluded ear simulator of IEC60318‑4 as used in former editions of IEC60118‑0 . For the measurement of the performance characteristics of hearing aids for simulated in situ working conditions, IEC60118‑8 can be used. For measurement of hearing aids under typical user settings and using a speech-like signal, IEC60118‑15 can be used. For the measurement of the performance characteristics of hearing aids for production, supply and delivery quality-assurance purposes, IEC60118‑7 can be used. The frequency range has been extended to 8kHz in this part of IEC60118 as opposed to 5kHz in IEC60118‑7 . Though the number of measurements covered by this part of IEC60118 is limited, it is not intended that all measurements described herein are mandatory. In cases of custom-made in-the-ear instruments, the data supplied by the manufacturer applies only to the particular hearing aid being tested.