In summary, there are several ways to measure both Balanced Armature drivers and assembled earphones using Balanced Armature drivers. Driver measurements made with the IEC 711 coupler are best for estimating their response when assembled into an earphone, but even then the response above 8 kHz is not accurate due to coupler resonance. Also, the earphone treble response is dependent on the ear tube tuning as well as the driver output so that the final result must be measured in the actual earphone. When measuring earphones a hybrid approach using two different couplers gives the best simulation of the response as perceived by listeners. Finally, for measuring earphones to 40 kHz to show compliance with the JAS “Hi-Res Audio” initiative, a special technique using a free air measurement was developed by Knowles.