This part of IEC 60318 describes an acoustic coupler for loading a hearing aid or insert earphone with a specified impedance when testing its acoustic performance, in the frequency range up to 16 kHz. It is suitable for air conduction hearing aids and earphones, coupled to the ear by means of ear inserts e.g. earmoulds or similar devices. The acoustic coupler does not simulate the human ear. It has however, an effective volume of only 0,4 cm³, which is small enough not to produce significant resonances in the coupler in the frequency range below 16 kHz. Therefore, it will load the earphone with a known acoustic impedance, which allows repeatable measurements with low uncertainty to be obtained on earphones used in extended high-frequency audiometry.