For two reasons it is difficult to estimate an earphone’s treble response from measurements of the Balanced Armature drivers. One is that the IEC 711 coupler cannot accurately show the output level beyond about 8 kHz. The second is that acoustic tuning of the earphone sound tube extends the high frequency response. This slide shows an example using Knowles tweeter WBFK. The graphs show the tweeter response measured with an IEC 711 coupler, and the same tweeter response when mounted into a properly tuned earphone.